on point one and three the problem is a dev can delete the download file and download page and itch doesn't seem to keep a backup to roll back to, not talking about them removing just the store page for a project they wish to abandon. it breaks the "and have offline access to it forever" thing you mentioned, yes i have done the recovery of purchase download links through email and several projects deleted the DL page, more simply don't have the DL file(s) anymore.
on the steam key thing that is why i specifically mentioned it only applying when it was included in the purchase, most easily where they checked the right box so the store page and dl page note the purchase includes a steam key; this is easiest to check since the dl page will have "your purchase includes a steam key" but there won't be any keys for the box to dispense
possibly also where the dev specifically promises keys to be included with a purchase or above a certain purchase amount though i would expect less enforcement of that with how easily a dev can delete the store page and thereby the easiest documentation of such a promise
on why, partially principal since it was part of a purchase, partially because some devs stop updating the itch version after getting on to steam, partially because it is insurance if the dev deletes the project on itch or if itch ever goes the way of desura (lost a lot when they went down)
on language that's why i chose the term "red flag" which is appropriate even in situations where they've tried but failed to complete a project, the word scammer is appropriate in cases such as where they delete a download, delete other files included at higher tiers, or don't follow through with explicitly sold keys.
All of the scenarios i mentioned are where users would have no option to get what they paid for, at least seemingly, it would be useful if the
"This project is not available for download
This game is not published and can no longer be downloaded."
page had a simple one button report thing to simplify and automate itch checking if they have a roll back, or if a dev emptying a dl page without putting a new file in a reasonable amount of time would trigger a potential problem flag, on the steam keys the easiest way would be if the steam key box had a button that would open a box where you could put a steam store page link so support could quickly verify there was a steam release without follow through. non-automated support should be a last resort
"keep a local copy..." lots of problems with that, not updated, need a huge server farm, and people would spend all their time trying to manually update and organize a collection without ever playing anything, back when i used to fill hd with stuff like that and make database sheets of which pc gamer disc had which patch for which game, etc. sounds simple, becomes insane