If you don't want to pay the TP prices, don't. Just wait until your guild is the appropriate level and purchase the amenity with plat. All you're paying for is the privilege to get them early, and while it'd be nice to have certain ones RIGHT NOW, it's not a big deal to wait. We did without ship buffs for 4 years after all
I buy most of my guild's airship amenities (with plat) and as I use them too, I don't see it as money wasted, or being forced to "suck up" the cost for the entire guild. The plat amounts are pretty paltry for someone who's been playing a while and has amassed a bunch of money.
Astral Diamonds aren't used to buy amenities, so I don't see how it would help if people could put them towards that use, and I can't see how you could donate TP since Turbine's been pretty staunch about those being non-transferable. Having a plat fund could work, so long as it was specifically earmarked or amenities to prevent misuse. Or maybe if you could put amenities in the guild chest for an officer to take out and use.