Not sure. If it's unlocked, then theoretically with "unlocking" having to do with unrestricted access for the IMEI (i.e. allowed on any network) it shouldn't be affected. I can't say for sure, but I would think firmware doesn't affect it. Have you factory reset since then? I would weigh your options and be prepared for "if" something goes wrong. Like I said, nothing should go wrong, but there's always a possibility, so be prepared. That's my insight.
Given that this is the U firmware, if it already works outside the USA then it should continue to do so. If it doesn't, just re-flash the stock firmware. No big deal there.
Nope, it won't work. I'm on AT&T and it doesn't add that option, so it would be software related. However, I would think that if you root and had the app package for the dialer, it would likely work if you know how to get it installed. There's also a separate package for video calling if I'm not mistaken. But other than that, it's gone. Hey, you get a feature that identifies (spam) callers though! I personally like that, but I digress.
No problem.