My brief answer is that we already have under-used tools to do this:
moderators can delete questions,
some users can vote to delete questions (and that now includes you),
more users can vote to close questions,
many more users can downvote questions, and
the Roomba does the rest.
This is the long answer:
Within our site the only option we have to ...
I can understand your frustration because I know it has happened to me on occasion too.
However, to get something implemented along those lines would need a convincing argument to be made to Meta Stack Exchange rather than Meta GIS SE and it looks like that has already been proposed there:
Prevent Questions From Deleted When Someone is Submitting an ...
There is an avenue which doesn't rely on our kind overlords at Stack Exchange to implement a deletion warning or lock on questions. If you have spent time and attention crafting a good answer that is worth sharing and keeping for posterity and those that come after: ask the question yourself!
(It would be worth spending time looking for related older ...
That question was deleted by the Roomba seven months after it was asked and before anyone had volunteered their time to offer an answer to it.
To avoid this automated process I recommend continuing to edit any question you ask, both in response to any clarifications sought by potential answerers and to include any additional relevant information. That way ...
I would agree with your motivation and have the same feeling at times. On the other hand,
The close reason itself
"to be deleted, pointless question".
seems to be too broad, and it can be easily abused by reviewers, shrinking a user base that is already small.
Could these choices be expanded to include "immediate deletion,
Perhaps there was a reason you did not find the other question.
So, besides what @PolyGeo answered, I suggest always try editing the duplicate question for readability and searchability (title, body and tags) if one thinks it is appropriate.
For example, usually titles starting with:
In QGIS plugin, ... or
How do I ...
are not the best for ...
Re 2: No, you cannot remove your close vote. However, close votes "fade" after a few days if no action is taken.
Re 3: Yes, in rare cases some off-topic (or patently bad) questions can produce great replies and thereby deserve to stay. (In this particular case, though, the cross-posting militated strongly against keeping the question here.)