Welcome to the forums!
Be sure to use the search function here. I know a few people have already done very similar projects, and you can probably learn a lot from them.
I would definitely get the NExT over the xEM. AFAIK there aren’t any real drawbacks, and you get the benefit of having an HF implant as well. NTAG216 is basically your standard NFC tag. You can scan it with your phone, put links on it, store contact data, etc. You can also use it with the KBR1 to log into your computer using your implant.
You can also pair it with tasker on Android to do pretty much whatever. I have one of my implants act as a KBR1 when scanned by my phone, another will connect me automatically to my college WiFi, and one will automatically pair my bluetooth devices when scanned. Obviously there are a ton of more advanced things you can do with tasker and NFC, those are just some examples.
On iOS I believe you can use their shortcuts or whatever it’s called to achieve similar, although significantly more limited, results. I don’t have an Apple device though, so I can’t say much about it.
Yep. This is going to be the one you want for that. I use my xM1 whenever I’m at a hotel that uses Mifare Classic 1k cards, and it’s pretty awesome. Just remember that not all hotels have powerful enough readers for the glass implants, so a flexM1 might be something you want to consider.
They can’t go in the same position because they’re both High Frequency. You can however put one in P0 and another in P1 in the same hand for example.
Totally! I have a NExT in R0 and an xM1 in L0. You can definitely do both.
Those are actually the ones I’m not as comfortable answering 
. I know there are a bunch of members here that are more than capable of answering that stuff though!
Ask away!