yes ofc, but if the treatment is the trip it will never work blinding it like this. If anything they should be looking for drugs that give comparable trips, with no potential for depression treatment.
I believe benzodiazepines have also been tried as an active placebo but I don't remember what the results were.
No idea about deliriants like atropine/scopolamine, but in low doses, they were traditionally used to spice up beer (hence Pilsen named for Bilsenkraut, black henbane), so I could imagine short antidepressant effect there too.
It does, and this observation led to the development of dedicated antidepressant drugs. There are several such cases where interesting side effects turned out to open a world of possibilities, including an anti-TB drug that also led to antidepressants, and of course Viagra.
"find something that has a similar trip but does nothing" is actually equivalent to the original problem.