mct16:
I don't see any evidence to suggest that he is living there on a temporary basis either.
Huh? That's circular logic - no evidence to support something doesn't automatically make a contrary position true.
When we met at Somerset House, for example, I didn't see anything to say that you
didn't live there - but that doesn't make any sort of case for me to decide that you
did live there...
Additionally, many people who regularly visit a temporary lodging will keep items there if it is convenient to do so - I didn't "live" at my parents' house after I left home at 18, but I had clothes, toiletries and such there such that I could have stayed there for weeks at a time without arriving with luggage.
mct16:
If Tintin did still have a flat in town then I think that we would have a couple of scenes there at least.
Why? Hergé could put characters wherever he liked, so again, not seeing something doesn't its presence "off-stage" untrue; if Wagner, for example, isn't seen at home - doesn't make the statement "We never see Wagner at home, so he doesn't have one...", true, does it?
mct16:
it would make sense for him to go back to his flat than go all the way to Marlinspike.
Again, false logic - maybe Haddock, passing through town, came to pick Tintin up for a weekend at Marlinspike, rather than for him to have take the train or bus, and they used the opportunity to take in a movie because there isn't a cinema in the village?
All this really goes to show is that the best that seems possible is surmise and speculation, but that there isn't really a concrete answer in the books as to the exact nature of Tintin's domestic set-up.