So rummaging through my spares box, I came across a useless-until-now adaptor that Leslie's sold me when I was trying to add a Matrix 1HP pump (before I understood the electrical risks with my kind of pool). I can no longer find it on their page, nor online, but just know that this adaptor exists and works beautifully! Behold:
On the right, the Intex vacuum adaptor. On the left, the Intex return valve it was meant for.

This is how they were supposed to work:

This is the reason I cannot use the Intex return -- check out how it narrows the flow inside:

So.... THIS IS THE ADAPTOR!!!:

It fits PERFECTLY withe the vacuum adaptor:

AND it is the EXACT thread as the eye that fits into the Hayward return:

One problem is that the adaptor is thick, and will not allow the sieve in. I think I might resolve that with a sock screen mounted elsewhere to protect my filter.
So now the single problem that I still have is to figure out how to remove that Hayward return eye from its socket on the actual pool -- seems pretty stuck in there. Suggestions are welcome.
The adaptor was sold by Leslie's as "this is what you need to connect your Hayward Matrixx Pump to your Intex 1.5" hoses" It goes from 3" to 1.5", and it has an O-ring on the 1.5" side. It is built as a solid block, with a ring that allows tightening by hand. If I find a link to it, I will post here.
Hope this helps someone! I added the many pictures because it might help someone figure out a different solution to this issue.