I had been planning to install the new porcelain tile over Ditra. Will that be okay with the remnant mastic there? If not, how best to handle it? Would it be better to install hardibacker, bedded in thinset over the mastic/subfloor?

Thanks for the answer. If I get the mastic flatter, is
there no concern about the thinset under the Ditra adhering
okay to the subfloor? Is there a special type of thinset
that adheres better to mastic, or a primer or some such?

It is about 380 square feet, so in any case, quite a bit
of scraping. A bit of it may also need some leveling
compound.

You need to use a modified thinset for all of this. Home Depot sells "Versabond," which will serve you well. White-gray, makes little difference. I wouldn't worry with a primer.

How thick will the total be, thinset-Ditra-thinset-tile?

Any idea what someone would have used in the 60s to directly attach tile to plywood subfloor, that matches this description?

Any further suggestions on removing this mastic? It sands quite readily, but that seems a bad thing, maybe even if wet.

You need to confirm that your subfloor is adequate for your new tile. You may find that you need another layer of plywood on top of what is already there, so don't kill yourself scraping up toxic mastic just yet :)

Not removing that existing mastic would add a bit of height under the plywood. Looks like the stackup (mastic, 3/8" plywood, Ditra, 3/8" tile, maybe some leveling) would hit 1", which means I'll have to add ~1/4" plywood under the rest of the flooring in the house (solid hardwood for some, cork for the rest so that needed more height anyway).