Hi, EPS.
The Janty warranty is a good one, but the rub is they expect you to use Janty liquid with their products. I suppose it makes sense if you're going to give a 6 month warranty on an atomizer that you want to ensure that people don't put bad juice in it.

In any event, if you look at the warranty book, it's pretty clear that using non-Janty juice could void the warranty. Of course, how that would be determined is another issue entirely.
Personally, I'd only order one extra atomizer because they have a 6 month warranty. Hopefully, the spare would hold you in the event you have a problem and have to wait on a warranty replacement. And since you and your wife will have the same model, perhaps one spare will do between the two of you.
As for the extra battery, honestly, I typically have at least 3 batteries for a product. However, I use the Janty Kissbox so much with the USB passthrough battery that the two they supplied have been sufficient. But, like I said, I use it mostly tied to the USB and I don't take it out and about much. If you plan on using it out and about a lot, then, yes, get an extra battery.
As for the cartridges, I generally just buy the prefilled cartridges, then when they get low, I refill them/top them off. I figure the cost of an empty is close enough to the cost of a filled cartridge that it winds up working just as well to get the prefilled.
If I want to try an entirely different flavor, I just vape it down then rinse the batting within the cartridge, dry it, stuff it back in and refill. There are all sorts of great tutorials on how to refill a cartridge--easy as pie. Promise.
I also sometimes make my own filling for cartridges when the stuffing starts to get too shredded and frayed. You can buy polyester batting at craft stores that works pretty well, and it's dirt cheap.
As for where to find the liquids, Janty has a fair numer. If you don't want to wait for them to get in stock, Diet Smokes carries Janty liquid:
Janty Liquid - Diet Smokes .com
And if you want to branch out beyond that, the world is your oyster. PureSmoker is popular, as is Johnson Creek.