Returning Player - Is anything valuable?

The Shreader's Talons in the screenshot are the prenerf variant of that item. (Not worth a whole lot to begin with but still valuable, but effectively worthless when customized.) Still nice to know though.

Because of that, it's probably likely that you have other prenerf greens elsewhere (either on other characters or on their heroes). Maybe you've seen it, maybe you haven't, but this thread covers all there is to know about those.

Worth taking a look: Greens

As for the other items listed, the birthday vouchers can't be traded, nor can the presents you choose with them. You just have to open them and see what you get. 6th and 7th year are always the recommended years for those. 6th arguably has more consistent value in terms of the tonics you get, but 7th has bigger jackpots in the form of the rarer minis in it: Naga, Oni, Zhed, Shiroken, Vizu.

The rest of the listed birthday minis are usually a couple k at most, or are just given away.

All voucher NPCs offer the same things, so no, it doesn't matter which one you talk to. For the vouchers/presents, correct. You can trade year 1-7 bday presents directly with other players. You can't trade vouchers or the surprise boxes you exchange them for, but you can trade whatever you get by opening those.

As for customization of weapons, unfortunately not. It's permanent.