Blanket statement: No one knows for sure what will happen in the future, and when. But...
It's almost certain there will be one program. And if it's going to take "a couple years", all questions about matching status across programs is likely moot. Today, status does match across programs, so the statement "I know SPG status does not give the same Marriott status " is not correct. Other than the fact that the tiers may have different names - they match like-for-like.
Lots of rumors, no facts.
I would guess that earnings toward LT in both programs will be factored into any combined program. Just be sure you need them - you need both points and nights for all LT levels, and likely will in the future. I know that points was usually the last item folks hit long ago, but seem to recall points was much easier to hit these days than nights (unless you're staying at $90 properties). It's been a while since I worried about it, so just check.