I'm certainly no expert in the subject matter, but I can speak a little about equalizing.
1. I would say No, it's probably not possible that you just can't equalize. Some people just have a harder time than others, especially if it's not something they're used to. I suppose anything's possible but highly doubtful in my opinion. Can you ever hear a popping sound in your ears when you swallow? What about when you move your jaw to one side or the other? Maybe these would be easier methods for you to try if the Valsalva method isn't working quite right for you. Like you, I have a history of ear infections from when I was younger. I had tubes inserted in my ears 4 times by the time I was 4 years old. I have lots of scar tissue in my tympanic membrane (TM) because of it. And I was really worried about whether or not I'd be able to equalize when I first started diving. And the first couple times it hurt a little . Not excruciatingly, but it wasn't exactly comfortable. But now that I've done it who knows how many times, it actually feels better when I do.
2. I would just the opposite. I would think younger people would have an easier time equalizing just because the muscles are "fresher" if that makes sense.
3. I'm not an instructor and again I don't claim to have any professional knowledge on the subject matter, but IN MY EXPERIENCE....yes it will get easier the more you do it. Like anything else, muscle memory certainly plays a part, so the more used to it those muscles get, the easier it will be for them to do what you ask when you ask.
4. I can't really answer one way or another on this because I spent plenty of time equalizing on land before I actually started my OW class. But I recently heard a PADI Asst. Instructor telling my son not to practice equalizing on land because he could damage his TM. He's doing the PADI Seal Team training right now. She said it's much easier on the membrane at depth because you have a greater force pushing inward against it so it doesn't require as much outward force to open the eustachian tubes. To me that sounds like a reasonable explanation. But I don't feel like I did any damage to my TM by practicing it on the surface.
If you got fluid behind your eardrum, though....you like ruptured it and had a pinhole sized tear in the membrane. I actually ruptured mine in the pool during my OW doing a pike dive and waiting until too late to equalize. It didn't hurt but I was able to blow bubbles out of my ear the rest of the day. Cost me six weeks of waiting to redo my class. I actually waited a year because I was nervous about it.
My advice is just keep at it. And make sure you equalize early and often. For me, I do it about every 2-3 feet. Granted I don't have to pinch my nose since I use a full face mask that places nodes in my nostrils to close them off, but still it's the same principle. Hope that helps!