90%+ rubbing alcohol and rock salt actually does a really good job, it just takes more elbow grease. Still won't be as clean as literally fucking dissolving it, but that's how I'd do it if I couldn't use acetone.

The gold standard for party bongs is shitpipes. Dunno about other places but for a Euro standard one, you need, bottom to top, 1x 100mm end cap, 1x 100mm sleeve (pipe to pipe coupling), 1x 100mm adapter to 40mm (the 100mm side should fit the sleeve), some length of 40mm pipe, some means to drill two holes, and two things not available at any random hardware store: a preferably metal what's it called in English the thing that goes into the water and some tissue to tighten that metal thing in its hole, wrap in a taper and twist in. Try to get an adapter that's angled, there's also ones with a flat step instead of (quite aggressively) tapering the diameter.

Completely and utterly indestructible at least when it comes to banging it up. In principle also fit to handle acetone those Euro standard pipes are polypropylene but I wouldn't push it. Tons of water in there, definitely bottom-heavy, and very light when empty.

You really don't clean it you let it get gunky, just replace the water, it's not like the gunk would dissolve into there. If you really have to, just spend what five to ten bucks on new hardware. Euro pipes should in principle hold up to acetone they're PP but eh, why. If it works it's probably still going to make the thing unsable for a week or so.

I always thought the same, and I've been toking for about two decades. For pieces my friends and I always prefer glass. If using a glass piece though, it's important to use vinegar once in a while (not just Isopropanol) to get rid of the chance of mould build up.