Hi and welcome to ECF,
I vape 90%+ vg and with the newer bottom coil tanks have never had to thin my juices. So I would suggest trying without thinning first. You can always thin later if you decide you need to.
Yes, your vg should be USP.
Please talk to the guys at that shop BEFORE you have them add nic, many vendors use pg based nicotine. That could be a problem if that is what they are using.
There are not many vg based flavorings, but also check out DIY Flavor Shack as well Dawn has started carrying some vg based flavorings.
While I have heard of people using these at times, no they are not needed. I have been DIY'ing for 3+ years and NEVER added these things to a juice.
TH is caused by your nic level, other things such as menthol, and some flavorings such as cinnamon can add to the th.
If you are open to trying dripping, I would suggest a cisco 510 1.8 ohm, I do suggest a sealed 510/510 adapter and drip shield. Not only does this give good flavor, but has a tighter draw than many tanks or rba/rda's out on the market and with what you are liking draw wise it worth checking out. It also will not matter how thick or thin a juice is with them.
While you are using pen styled devices I wouldn't go below the 1.8 ohm.
Higher levels of PG causes throat hit too.