#3. The idea is to tie the account a specific device (such as an iPhone), by generating a unique signature in the app. For this to be enforceable, logins outside the app cannot be permitted -- otherwise signatures can be forged.
#1. I still think #2 is a more important barrier, but I don't see a huge downside with this either. You didn't explicitly say this, but I think you're making the point that troll accounts can never be completely eradicated. But you can make it significantly less convenient for them, while minimally inconveniencing most legitimate users.
#3. I get the general idea, but I don't see how it's substantially more secure than requiring 2FA. I mean, GMail, Dropbox, AWS, etc., seem to think 2FA is sufficiently secure for very sensitive data, and they allow interacting through the web browser. Plus, at the end of the day, you'll need some mechanism for adding new devices, which will effectively work like 2FA – I presume.
I don't entirely follow points #1 and #3.
Re #1: There are myriad ways to get a temporary phone number, beyond Google Voice, Twilio, etc. For example, I can just use an anonymous pay-as-you-go phone. What would this accomplish beyond creating some slight inconvenience?
Re #3: I get the need for strong security. You could require multi-factor auth. But what does disallowing participation through a web browser get you that 2FA doesn't?
These are sincere questions. I'm not trying to be snarky, just understand what you're saying, because I agree there's room for improvement in this area.