Some of these are issues of personal preference, and since there really isn't much "try before you buy" with smartphones, it can be hard to know what works best for you. VZW does have a 14-day return period, however.
1) All Android phone carriers add crap/bloatware to their phones, or so I've heard. The do it because the app makers pay them to do so, and supposedly it reduces the cost of the phone. (Like they weren't going to get that back from our monthly fees anyway.) In current Android phones, you can disable preinstalled apps, though I don't think you can remove them entirely. You can remove their shortcuts from your home screen, though, and create folders in the Applications area that will hide them.
2) Don't really have any experience to comment from here.
3) Which phone to buy absolutely depends on your preferences for looks, the way it feels in your hand, and how you like to work and/or play. Read up on the phones and make sure you know what you're really looking for before you go to a store. Do as much trying out in the store as they will let you. Don't listen much to what salespeople say. They generally try to sell you a more expensive phone, which may or may not meet your needs.
4) As for VZW vs. T-Mobile, AT&T or Sprint, it depends partly on where, how, and when you'll be using the phone. VZW does have a larger network for both voice and 4G data. We get reception at a rural place in the western North Carolina mountains on VZW that we could not get on AT&T. But our daughter says her Sprint-network phone works there too. Ask people you know who use their phones where you intend to use yours and see what carriers they like or dislike.
One last thing: this is a forum mainly focused on problem-solving, so what you're going to hear here is mostly people having problems. It doesn't necessarily reflect the overall user experience, though it may show where some special weak spots are.
