I would check the formatting on the lookup field and make sure it’s one of the first four. If not, the lookup thinks your number is a string (probably). I just tried using MIN with a lookup (currency original field) and it worked fine.
Not sure… that’s a little trickier since Airtable handles arrays relatively strangely. I think people have solved that problem before in other threads. I think it involves some combination of the ARRAY functions.