Counter idea: Run a meta contest for a 30 word or less mini-requirement statement.
The "Welcome modal for new askers" s too wordy IMO.
We all have ideas of what new users need to read.
It can't be long (few read it) nor too short (uninformative).
My comments per your short list
Ensure that you are the author/maintainer of the code
Authorship is ...
I don't think linking directly to the guide to posting is a good idea.
It's fairly verbose, it's quite literally a wall of text. Whilst this is undoubtebly good for people that want to get the most out of their post it's just going to make the average Joe run.
It doesn't actually tell the user anything that is off-topic. The link only contains site policy ...