2. The idea behind the suggestion to change servers, which you see reasonably frequently, is that if you always connect to say, server 31 in Finland, that's a pattern that could be exploited to track your activity. For example if you have five socks on KiwiFarms, and you always connect from the same VPN server, that makes it almost as obvious to the sysops that all of them are you as you always connecting from your home connection would be. And at a higher level, if you're doing illegal activity and that's your only protection, in theory the host country of the server could monitor it and do traffic analysis.
So better to 'favourite' a safe country or three, and always connect to a random server from one of those.
3. I wouldn't worry about that so much. But it makes sense to avoid servers in your own country and ideally stick to those in relatively free countries- Serbia, China (Hong Kong Special Autonomous Region), Romania, etc.
4.
@Smaug's Smokey Hole is entirely right about how you would want to separate anything you did (whether on KF or elsewhere) that could count as a crime in the US from your 'normal' traffic, because otherwise the pigs could correlate things like IPs and times for Google account logins to legally obtained information about where your traffic to the website was coming from.
If you just want to ensure that all your traffic goes through a VPN, recent Android versions have an 'Always on VPN' option. Alternatively you could use the mobile Tor Browser for anything you consider sensitive. Or a VPN and the Tor Browser.