Hi Darren,
I was the one who initially started this whole thread so let me give you my experience...
1. I didn't have any issues with the ID verification process with IDNow; but I was in a room with pretty bright lighting and the agent did ask me to tilt my US passport left right, up down, etc. and she gets to control the flash of my phone to shine on my passport. It was relatively painless, but like you said, YMMV, make sure you use a decent smartphone. :-)
2. I happen to have a European cell number and had no issue getting the security code, didn't even think about using my US cell number - but good to know!
I use TransferWise (a British company) to transfer money from my US bank acct to my new N26 account. Takes maybe 2-3 days tops. When you sign up for TransferWise.com (they also have an Android/Apple app), you take a picture of your driver license (as proof of identity they store in their system), specify how much you want to transfer (in US dollars) which they then convert to Euros (with very little commission - i haven't found anything lower elsewhere, not even PayPal), and as you "lock in" that exchange rate, you have about 24 hours to deposit your specific US dollars. Here's the beauty - you can do a direct debit from a US bank (and Transferwise does not charge a fee for this), OR you can do wire transfer to a Transferwise's US bank account (Wells Fargo I think), in US dollar denomination. So you're never doing any international debit/transfer.
Depending on your bank, you may have free outgoing wire transfer (or nominal fee) - but this whole thing beats the heck out of trying to do an international wire transfer PLUS having your bank do the currency exchange (which you can bet your bottom dollar that it'll cost an arm and a leg!)
In any case, hope this helps, enjoy your new IBAN number which comes handy if you do a lot of eBay transaction with someone in Europe. Also remember, they don't send their debit card internationally (as far as I know), so I hope you have some kind of arrangement to get it. I haven't used their debit card state-side, but I will when I go to Europe.