Here I've asked the community to verify if my alternative proof is valid, but I now see (with the help of Siméon) that it is nonsense. This question may clutter the site, so I was wondering if I should delete it?
I disagree that you should delete the question. It is a perfectly fine question, and, as with any proof-verification question, there is a chance that someone will learn from your incorrect proof or will come up with the same proof. Errors are a wonderful way of learning!
Deleting your question serves noone, but leaving it up at least allows it the possibility of being useful.
A slightly different, (meta-)argument is as follows: The proof verification tag exists, hence it must have a use. However, post of the form
Q: Is my proof correct? A: Yes.
are just noise - unless the proof is very elegant, noone cares. Noone else will learn anything. Instead, posts of the form
Q: Is my proof correct? A: No, because...A way to fix this would be...
are great. People may learn from your errors! Hence, posts of your form are good, and are a justification for the proof-verification tag.
When you yourself realized the source or basis of your error after posting was wrong (fair enough, no one is above error, errors are in fact paths to progress.. even at times culminating into full success), honestly state how the error arose. Give notice that you intend to delete it after a week or so. That way it halts further time waste of those who may have taken your question too seriously and delved into the deep for further investigations. Truth is fair to all, including yourself.