Regarding: question editing
It would be better to restrict the modification of a tag question. At least by allowing only to modify one at time and also require to provide a reason of more than 50 characters
First of all, the only thing that will happen as a result of preventing users from modifying more than one tag at a time is a bunch of unnecessary revisions. If someone wants to modify the tags, they aren't gonna stop just because they have to do one tag edit per revision.
In response to requiring a 50 character description, this just isn't going to help anything. It's generally pretty obvious that the editor is just adding relevant tags or removing irrelevant ones. In the cases where a description is required, very few to none of them will require 50 characters, and there's no reason to force people to make them a lot longer than necessary.
Why should this be done?
What if someone uses totally wrong tags for a question? -> you have to change all tags
What if the reason for a change is obvious? -> you don’t need an edit reason
Also all edits are open to review as the post is bumped to the front page. Abuse is therefore quite unlikely and dealt with quickly.
If you disagree with a change, roll back the edit or edit it back.
There is really no reason to do this.
There's already a feature that exists to tell you when a tag should and shouldn't be used: tag excerpts. You can see this by hovering over a tag, and many tags also have extended guidelines in the full wiki. For example, here is the feature-request wiki.
If the editor doesn't feel like this is enough explanation, they can leave a comment (either a proper comment or as the edit summary). If someone edits your question (or you have 50 rep on the site and it's someone else's question), you can use @username to ping the editor about their edit.
What you're suggesting would end up needlessly wasting editors a lot of time, time that could be better spent on doing more important things: answering or asking questions, making more edits, IRL things, etc. This restriction will also prevent some people from making edits at all. There is always going to be more content that needs attention than editors, so it's very important that editing be as easy as possible.