I have it set to self-employed without any problems. Never had problems with proof of funds, I would use a Revolut or similar to fund the account from the same IBAN. I always fund in USD to not have to swap EUR for USD on IB itself.
It depends on your nationality and amount of funds, but I never had problems with depositing a few K per month and withdrawals but I am from a rich Western country and had an inheritance when I became of legal age and opened the account.
I would just tick boxes for tons of experience, tell them the money is from inheritance and income. If you plan to fund the account with more than 100K you might get asked for source of funds, I am not the person who can advice you on how to avoid that check in the first place. It really depends on the situation.
Also not able to enlighten you on this. If you are from a low-income country and 26 years old and show up with 80K they might want to know how you earned it.
All I can say is that IB is a world-class broker which I have never had any problems with whatsoever. And don't stress about those questions too much, you can lie all you want and it won't matter. IB is not the type of broker to freeze your account and steal the money.