I'm sure this varies by state, but is there any federal licensing required to either trade other peoples accounts, or trade other peoples money in your account?
Checkout IBs Friends and Family, that's about the best info you will find and you can call them on the phone.
Here is a good overview - I do not, nor would I ever want or need to manage OPM. https://greentradertax.com/wp-content/uploads/2015/04/Trading-Other-Peoples-Money-06042015.pdf
For sure. Lot's of bullshit that distracts you from the markets and actually making money, so unless it really changes your equation, it's better to avoid hands-on dealing with investors. Also, probability-adjusted you can probably make more money as a trader/PM for an existing OPM setup. I don't think the OP is anywhere at the stage where he can be expected to successfully run OPM, but we can keep this up as a theoretical discussion.
Depends. If it's friends/family, I don't think you need anything. They'd just have to sign off that they give you permission to execute trades. As far as professional money management, I would imagine you'd at least have to have a Series 65.