Learn Swedish: Vi vet inte vem som lämnar sprayburken. - We do not know who leaves the spray can.

In Swedish, when an indirect question acts as an embedded clause and the question word is the subject of that clause, you must add the relative particle som. Since vem (who) is the one doing the leaving here, it is the subject, so we must say vem som lämnar. If you leave out som, the sentence is grammatically incorrect.

If the sentence were 'We don't know who he is leaving behind', would we still need som?

No, you would just say Vi vet inte vem han lämnar. You only insert som when the interrogative word (vem, vad, vilken) is the subject of the action. When someone else is the subject (like han), vem is the object, so som is dropped.

Where would inte go if the sentence meant 'We don't know who does not leave the spray can'?

It would be Vi vet inte vem som inte lämnar sprayburken. In a subordinate clause (like this indirect question), sentence adverbs like inte must come before the verb. This is often taught as the BIFF rule: Bisats, Inte Före Finita verbet (Subclause, 'inte' before the finite verb).

Is it normal to glue an English loanword like spray directly to a Swedish word like ?