Having re-read your original post, despite what you say, it really sounds like you are not appreciating how long it should take a boat to stop in reverse. "several additional yards" after you put it in reverse is perfectly normal.
When I dock the boat, the whole thing is accomplished at idle engine speed. That is how fast you should be going in the marina anyway. It would be a good exercise for you to try docking without use of high RPM.
There is a question about whether your transmission is slipping. This can easily be confirmed in the dock, providing you have reasonably secure docking lines. Go from forward to reverse, and observe the prop shaft. Is it rotating at engine speed? Is it rotating at the same speed in reverse as in forward?
Have you checked the transmission fluid? The linkage adjustment?
The overheating problem is something else. Treat one problem at a time. Suggest the overheating first.
I am seriously concerned that you're a danger to yourself and to others. The one thing you really need when you're inexperienced is an engine you can rely on (well, a boat that doesn't sink is important too
). Now is a good time to learn all about the engine, fix everything that's not spot on, so you can have faith in it.