Sounds like you are trying all sorts of ideas to get something to work. I give you credit for trying, but I think you should set some basic rules.
It's bedtime, you have to stay in bed. Implement a punishment.
If he truly has to go potty, let him go, once. If you can't tell, then set a time limit. If he plays around, put him back to bed. Don't chat with him while you wait. Stand outside the bathroom door.
When my son was 4, he had to sit on a chair in the kitchen for 2 minutes if he got out of bed or came out of his room. After a few days of that, he'd decided that staying in his bed was much better.
It sounds like his multiple requests are just tricks to stay up longer and be with you. So, maybe if you praise him verbally for what a good job he does, each time he stays in bed, you will get somewhere.
You just need to set some limits. If he has done all the bedtime routine, had a drink, etc, then there is no reason to get out of bed. He just needs to calm himself and get to sleep.
So, whatever you do, make a new stay in bed routine and stick to it, don't change it. I would guess that is probably causing him confusion and the ideas that he can do things to get you to let him out of his room.
If you decide to use the gate, keep it on all the time, not just when it's the last resort. At 3, though, I think maybe once he gets better about staying in his room, you wouldn't need it, unless it's for his own safety.
As far as waking up the other sibling, just remember this won't last forever. The sooner you stick to your guns, the sooner everyone will go to bed on time.
Oh, one last thought. If he truly doesn't seem tired at bedtime, maybe change his bedtime to 15 to 30 minutes later for several days, and see if that makes a difference. I wouldn't go after 8 though, but that's me.
Best of luck, I hope you get some peace soon, I know how frustrating that can be. My 2 yo has decided she's going to climb out of her crib. So, we are in the transition to letting her be able to get in and out of her bed now without help. We took the rail off and put on a side guard. She has been doing surprisingly well for the latter half of the first week. We do gate the door though, that has helped the game of running down the hall. =)