Your diy platform idea sounds good. And screws are fine. It is hemp mat on top of the house and shelf
It's safe if chewed but none of mine have chewed it. Occasionally they chavel it a bit or pull a bit off. I put it to cushion any falls if they happen to monkey bar across the roof when you're asleep at night. But also as a bit of insulation on the house in winter. I take it off in summer usually. On the shelf I've sometimes used it to protect the wood if I haven't plastikoted a shelf.
Rodipet sell the best hemp mats. Some of them shed too much. And some are too thick to cut. I get the 100cm one and just cut pieces off it to size. Which leaves spare if you ever need to replace a piece.
RodipetŪ Hemp Mat 100 x 40 cm
7cm diameter is fine for tubes. Straight are better than angled as they're more likely to get stuck in an angled tube.
I think filling the cage base with bedding is fine. Having the bars, with things attached, and shelves and roof runs etc, adds to enrichment as much as depth of substrate.
Hmm about the door being loose on one side. Is it easily fixable? If it's a problem or leaves a gap I'd send it back for exchange.