I've invested a lot of time into 5e, but I'm sad to say that we're migrating to PF2e in the next couple months/years. I have two 5e campaigns to finish my saga, and I don't want to convert them to PF2e and run them in there because that would be bonkers tedious. Too much work for too little gain.
After they're over? I've already got Foundry set up for the next campaign in PF2e. I'm surprised with myself, honestly. I figured Paizo coming out with 2e was a bad move, but it seems I was incorrect.
The problem with DMs Guild — and I speak from experience of publishing through several edition changes — is that pre-OneD&D material sales will drop off a cliff. Even if they are technically still compatible (as in 3E-3.5) people won’t buy it if it wasn’t made specifically for the current edition. Been there, done that!
Depends on the product, and how easy said changes actually are to make. Adventures should certainly be very simple; entire alternate rulesets that depend heavily on mechanics changed in 1D&D, less so. But we won't know for sure until we see the final version of 1D&D.
That said, I expect you'll see some creators making an attempt at minimal and/or flawed conversions and just claiming compatibility, as you suggest. (Legendary Games seems to get away with this for their Pathfinder conversions...)
Not putting out an official One D&D srd will have exactly no effect on the 3rd party publishing industry.
Most 3PPs don't actually care about what is or isn't in the SRD. I freelance a fair bit in the 5E space and honestly they are always trying to use stuff not released under the SRD. And they can because in the end all the SRD does is alleviate copyright rules for specific expressions of specific rules. You can't actually copyright rules so few companies actually care and I have yet to hear of WotC going after a 3PP company that included stuff outside the SRD.