Not sure what you mean exactly by "counterparty with proof of stake on an own chain (or nxt or ppc)" ... https://counterparty.co/faqs/can-counterparty-work-on-blockchains-other-than-bitcoin/

Counterparty could probably work on a hybrid POS blockchain like PPC.... the key is really around being able to encode the data into both the POW blocks and the POS blocks. I'm not 100% certain of the state of PPC's multisig support (and it probably doesn't relay OP_RETURN currently, as that's just coming around for bitcoin itself). However, as long as we can do multisig into the blocks, then in concept counterparty could be made to work with a different Bitcoin-based blockchain. Note that there are other technical considerations that would need to be taken into account as well that could have an impact on the implementation, such as block timing.

With things like NXT, that's totally up in the air, as it's not based around Bitcoin. I don't know enough about it to say one way or the other. Probably just simpler for NXT to develop that kind of functionality natively.

With regards to your GUI question... that is the whole point of a web wallet, there *is* no installation. No counterpartyd to set up and install. You simply go to a webpage, generate a passphrase, paste it into a text box, and click login. Takes all of 5 seconds. Nothing to save or worry about beyond your pass phrase (which *is* your wallet)