If the next Xbox has Steam does that mean it will have mod support

You have to check the game listing page if it does

Not all games support mods steam doesn't support mods bit it doesnt prevent you from using them

I.for example use ff7 mods fro. Nexus not supported by steam

So does that mean this has ended in a 'steaming' pile?

That is a grey area. Seeing as it is a game from Steam and not Xbox, where only certain games has in game mod support like Fallout 4, Skyrime Special edition, Farming Simulator 17 and 19.

Really need to check on the streaming service you will be using to stream to Xbox aswell what they have setup regarding mods.

The majority of posts on the first 10 pages of the Rewards support forums are by a rewards mod. That seems to be the case pretty much every weekday afternoon. So I wouldn't say that the mods are ignoring that forum. Now it does have different mods than the
support forums, but that doesn't mean it doesn't have mods.

It's the publisher who has the last say on the game prices of digital downloads on the Xbox Marketplace, not Microsoft. Just because a PC game on steam is x amount, does not mean its Xbox version has to be the same price.