If I may add a third one, is the PhoenixCoin platform has a fund history? Like a proof showing a token successfully mined or moved somewhere? If they have, maybe you can use that.
3. PXC is a Bitcoin fork made in spring of 2013. Probably one of the first 20 coins ever made. Sure there's a working wallet, block explorer, etc.
No, I mean do you have a transaction record within the wallet itself? I looked at PhoenixCoin's website and learned that they have a desktop wallet and I assume this is what you use. Within the wallet app, s there a tab that shows transaction history? If there's any, a screenshot of it is probably a proof enough --although signed message is also a strong one, if you ask for my personal opinion.
It is, but wallet screenshots don't actually prove anything. Import any address as watch-only and it will look like you own it. I mean the complete transaction history will be there. However no private key, nothing can be spent.