1) Yes and no. For example, in case you live in LA and pick the proxy from the same city, more likely you will get a proxy from another ISP/cellular provider than your current. In addition, any way you will have to rewarm your account if you would like to run it on some automated software because of software emulates a new hardware device ID, which has no trust with your account.
2) Originally rotating proxies were able to help keep the actions, when IG tried temporary block the action on a particular IP. That's why a simple rotating trick fixed the issues. Now its also working, but the mobile proxies is not a holy grail anymore.
3) I'll hold myself back here.
4) Directly depends on the account trust level, your actions, skils unfortunately and the software. In case of software could be detected - it doesn't matter hows good you are - anyway, u will get the problems. But in general even right now with the software its till possible to follow more than 6k users (once again depends on account to account)