since I want started learning any%, I have trouble with one thing: sometimes, the level doesn't count. Why ?

Edit: HOW CAN YOU ENTER THE PIPE ???

There are simply 2 different level clear flags in the game: supposedly, 1 for beating the level by taking the secret exit (the flag still exists in the code even if a level doesn't have such an exit) and one for beating it normally. Only the latter marks the level as completed.

With Super Mario (mushroom upgrade) you have to do a little jump on the pipe to be able to enter it. (Imagine there's an invisible switch ~2 tiles above the pipe and Mario has to hit it to open up the pipe below. With Fire Mario (flower upgrade) you just have to get to the pipe quickly and without falling too far down on your way to it and then enter it centrally or a little to the right of the center. Since you're on emulator, there might be different factors that we don't know about yet. Sorry for the very late reply!

Is it possible to use the pipe 2) from the previous post consistently? Sometimes i just come into the pipe and sometimes Mario just will not go in and is doing things like sliding to the left

I have found a good explanation about the behaviour of the first row in the out of bounds area

Yes. If you stand on/near the pipe, jump and land on the pipe tile holding no direction on the d-pad. You should be able to enter it nearly every time. The important part is to not hold any direction while landing. As soon as you break a block above the pipe, you'll be unable to enter.

2 minutes into that video:

oh wow, I always thought the pipe bug only works on version 1.0 and pipe death works on any version. Looks like I was wrong (I tested to make sure).

The $A2D5 address being an "unused debug flag" sounds plausible.

Wrapping the screen upwards (i.e. entering glitch world from below) seems indeed impossible. I had tested that many years ago and couldn't do it, although you can interact with the last 1 or 2 rows (as far as I remember).