I've had every problem described here except the "shifted button" issue. Some problems such as a blank screen or other strange behavior that I've had has been remedied by re-applying Noalox on the threads. In other words, the ground contact in the threads was flaky and needed to be cleaned and Noalox re-applied.
My biggest problem has been the flaky button with misfires and inadvertent entry into the menu. The button has been a real PITA for me. My Dad bought one on my recommendation after I had it a few weeks (before the button problems started) and both of us eventually ended up with the exact same problem. In my opinion, the button used in this PV is cheap and doesn't withstand the constant use over time - not a good choice by Segelei for use in this PV. Cleaning or blowing it out hasn't helped me or my Dad much, which doesn't surprise me since once I took mine apart and saw that the button doesn't have any openings that air or cleaner can enter to get to the contacts. I decided to see what I could do to fix it and bought a backup Vamo V3 while I was working on the Segelei Zmax.
Anyway, I took the top cap off in a vise using some leather and vise grips to keep from scratching it up as much as possible and working back and forth on the top cap as it slowly came off a little at a time. When it seems like it might be about to come off, use caution so that when it comes off you don't yank the wires out of the circuit board. Then I attempted to find a suitable replacement button to order online based on the original switch characteristics. I tested the switch with an ohm meter an it's a Single-Pole/Single-Throw (SPST) surface mounted tactile switch. I have a Bachelor's in Electronics Engineering Technology with 22 years of electronics experience, and I only mention it so nobody thinks I'm some quack shooting in the dark.
I also took measurements of the solder contact spacing for the soldering pads (5mm) as well as the body dimensions (12mm long x 12mm wide and 5mm tall). After looking at several possible replacement candidate switches on Digikey, I chose a replacement based on those dimensions as well as the durability (lifespan based on the number of switch presses). The switch I chose on the Digikey website was part number
450-2051-1-ND ($2.67 plus shipping at the time of this post). The switch's datasheet reported the durability to 1,000,000 cycles (key presses), which was better than the 2 other possible switches with matching dimensions. It was also the only button that was "sealed" against foreign contamination. I don't know why the original switch began to fail, but I'd rather not have to open my PV ever again after this fix so I want a sealed switch. Like I said, the replacement switch dimensions were the same except that the height of the switch was lower at 4.3mm (none of the possible switches I found had the exact same height as the original at 5mm). Because of this, I needed to build up the underside of the external plastic button on the Segelei so there wouldn't be a lot of button rattle. I used a small silicone "sticky foot pad" sometimes used for feet or for deadening the sound of closing cabinet doors. This seems to have worked out well so far and it was probably around 1 mm that I added or possibly slightly more (I remember there being some extra slop in the original button which it no longer has). I'm sure that there are other/better things to use as a spacer that would work equally well or even better than what I used.
De-soldering the old switch was a bit tricky since both sides of the switch have 2 contacts. I put the small circuit card in my bench vise (gently) and used a long tip soldering iron that allowed me to heat both contacts on each side at the same time so I could lift them off the pads on the circuit board simultaneously. I did this for each side of the switch. I wish I had the correct soldering tip for de-soldering multiple contacts, but this worked relatively well (but required a steady hand). If you decide to attempt this, just be cautious that the solder on both contacts is melted before you put any force behind lifting them off. They will lift off pretty easy when the solder is melted, so don't pull the pads off the circuit board if the solder isn't melted yet (also don't apply too much heat to the board or you may damage other components). Once the old switch was removed, soldering the new switch on was pretty easy. Don't worry about the orientation of the new switch (it can be soldered on in one of two orientations). You can choose to orient it one way or turn it 180 degrees. Either way is perfectly correct. I recommend not over heating the switch (allow it to cool sufficiently between soldering each of the 4 legs). The soldering temperature limits for the switch are shown on the datasheet at Digikey (above link) if you need to refer to it. You can damage this switch if you don't pay attention to this and apply too much heat. Just be quick about it and remove the heat ASAP and you should be fine (no excessive paranoia required here). You can also use compressed air to cool it faster if you choose to.
Putting it back together (after the circular black disk and clear button are in place) put the glue in the tube first around the area that the black semi-circular plastic support on the circuit card will rest against the inside of the tube. Old glue will be visible here and some of it may need to be scraped away first as you see fit. Then insert the circuit card into the tube while trying not to push the wet glue down further into the tube and then seat it into the glue once the card is all the way in. Also try not to get glue above that where the top cap will eventually seat along the top of the tube (wipe off any excess along the top if you need to). I recommend letting the glue dry completely before putting on the top cap so the circuit card won't shift. If you want to test it before you glue it in (not a bad idea), do it before you apply glue because the battery will slightly push all the guts up and out of position (but it will still work). Go easy on the battery spring pressure for this reason. The top cap will also need to be inserted firmly for testing but where you can still remove it later to glue the card in.
I'm happy to say that my Segelei Zmax V3 now works great! I need to say that this particular button is not as "clicky" as the original was and doesn't require as much force to depress. Some people may not appreciate that since it might be easier to fire by accident if it's in your pocket, etc., but it's not overly sensitive and honestly, I kinda like it. It's super easy to fire in comparison to how I had to squash it down before just to get it to fire properly (sore fingers for sure). Definitely a good trade-off in my opinion. I feel enough of a "thunk" under my finger when I press it to let me know the switch is engaged and it has been operating perfectly every time. What a relief! Now that I have the Segelei working good again, I've made the Segelei Zmax my "run out the door" PV using a smaller 18350 battery in it's shortest configuration.
I hope this helps anyone else out that might have been considering attempting something similar but just didn't know what replacement switch to buy or where to buy it. On a scale of 1 to 10 for difficulty, I'd say this averages a 3 or 4. It was really not that tough to do and should be do-able by anyone that's relatively handy and that has some basic soldering skills. The toughest part might be de-soldering two legs at the same time. If you can't get your hands on a long tip iron, then maybe a friend can help you out with a separate soldering iron (4 hands are better than 2).
I'll be sure to post back if I notice any problems over the long term with this replacement switch, but so far it has been working perfectly. I'm certain that this switch is of much better quality than the original that came with the PV.
Here's a few pics:

Outer button with the spacer to make up the difference in switch height.
There's no button rattle to speak of.

New switch installed

Old Switch