1. I wouldn't bot any more than 4 accs on a single ip, using a proxy would help if you want more bots
2. A good vpn shouldn't add any lag to runescape
3. Yes, if they flag your ip as a botter, they will flag all the accounts and they'll most likely get the ban hammer
4. Not if you don't bot on it, but be careful if your going to play on a flagged ip
I recommend getting VIP because you'll get to run more than two bots, and you'll get the mirror client, while a little buggy, is still amazing for keeping the ban rates down.