Box art is allowed to be used as much as you want. Encouraged even. If you can use some tiny detail in the piece to figure out where it goes overall, I think that's awesome. For me, you're building the picture moreso than connecting weird shaped edges to each other. The weird edges are just confirmation that your piece belongs there, not the puzzle itself. Other puzzles that focus on the same shaped or specially shaped pieces are a different kind of puzzle(like those triangle puzzles), but not jigsaw.
No timers, just a general "it took me x days/hours". Not looking to break any records, just relaxing.
Stuck together pieces varies. If it's a huge clump, I'll break it up. Little 2-3 bits I'll keep as bonus starters. Most I'd probably allow is a blob of 8 pieces. Anything bigger feels like I'm missing out on the puzzle. Unless it's some 5000 piece monstrosity... So small amounts in context, yes.