Re: Difficult uke or bad technique?
It sounds like mostly a case of a bad uke to me, particularly given that you've been clear with him about what you need. At the same time, good ukes don't (usually) just happen, and people who are strong and can grip like iron don't turn into good ukes unless they've had someone explain to them what uke's role is. Uke's role is to execute the attack they're supposed to, with a speed and intensity (strength, if you will) that gives nage something they can work with, given their level, to execute the technique they're supposed to. Those are the conditions that allow nage to train. If you're acting in such a way that nage can't train (and therefore learn to handle attacks with greater speed and intensity), you're a bad uke -- but only if you've had that uke role explained to you in that way. So I'd put it back on the sensei and sempai, at least to start with. If he has received that explanation, and he either rejects it, or thinks it doesn't apply to him, or thinks that it makes training "not realistic" (or, worse yet, he uses the "not realistic" judgment as a fig leaf for a self-indulgent ego-gratification game of "You can't handle my attack, I'm better than you, neener neener neener"), then he's a bad uke.
Dealing with a bad uke can be very tough, depending on the dojo. My sensei really frowns on nage who color outside the lines -- for good reason, because it's an easy way for a bad uke to get hurt, and also it can become a convenient excuse for nage to be lazy and fail to learn to deal with more difficult attacks. But then, when he calls people up for ukemi, he's absolutely devastating on any kind of "bad uke" behavior: attack him in such a way that it would be very hard to do the technique he's "supposed" to do, but easy to do another technique, he'll go with plan B and uke will get schooled. So we don't get people with ingrained "bad uke" habits like that. I guess I've circled around to the same point, and it's back on the sensei. If your sensei doesn't mind if you do a different technique, that's what I'd do, I guess.