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
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.
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?
it would make sense for him to go back to his flat than go all the way to Marlinspike.
Again, false logic - maybe Haddock came into town 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.