No, it is no more difficult too pee with an erection than it is without one. Where that myth stems from is that with an erection, you can not point it down at the toilet because that kinks the urethra and then you can't pee. If you let it point where it wants to point, it is no different.
If you have any amount of diaper changing experience with boys, you have probably been peed on, and it was almost certainly from an erect penis. Babies are erect more often than not when they pee.
Sit too pee? No normal guy does that. It would not make sense to use the extra effort to disrobe too the point that would enable sitting down too pee. I have only ever heard of one male that did this on a regular basis, his wife asked me if that was normal for guys too do. This guy was one of those femmy peculiar acting guys. Other than him, never heard of a sitter.
i don't even think it's possible to pee with an erection, every time I have an erection and I want to pee, I just have to try to calm myself down and get less turned on to reach a level where peeing is possible. it can take something like five minutes.
I've never had a problem peeing when hard (other than aiming). Likewise, I've never had a problem cumming when soft (with a strong enough clamp!) But Man's Best Friend is an odd body part -- when soft, it's designed to pee, and when hard, it's designed to pump. So if I ever find myself having to pee at an inopportune time, say, on a highway with no available rest stop, I just get an erection and the urge to pee goes away completely, because my penis has been "transformed" from a pee-er into a pumper! Once I get to a rest room, I can easily empty my bladder whether or not I'm still hard.
Yes it is possible. Its harder to do and much harder to aim. If I bend over the toilet (like put my head against the wall) I can do it. There's a real increase in velocity though and it splashes more. It would be interesting outside to see if there's an increase in distance... but sitting down yes, I think that's a situational thing. If a guy has ever tried standing up to pee on an airplane or on a boat thats rocking or had a wife with a rolling pin he sits to pee when necessary. My wife doesn't turn the lights on in the night and complains about falling in when the seat gets left up so to avoid this situation and forgetting to put it down, at night i just sit too.
Yes it is hard to pee with an erection. Normally you have to wait for it to go away. I find though if I have a "morning glory", which means my bladder is full and putting pressure on the prostate gland keeping me very hard... I need to pee to make it go down or it's very embarrassing. So I lean right forward and try to push it down so it points down. It's not easier but such a relief when you're able to pee!
And I've never peed sitting down, but I read somewhere once it's a good idea to do that one first dates so you don't pee on the girls floor or toilet seat or anything accidentally and put her off! *LOL*