Jyn Erso Shards???

The characters in the guild store aren't likely to be moved or added anywhere else. The only times I can remember characters being moved (but not removed) on a table were when CG wanted the Ebon Hawk (I think) and relic materials on the later cantina nodes, but there were already characters there, so they were moved to the easier stages. I don't recall store characters ever being moved out of a store.

So you'll just have to save guild currency and check the store every refresh.

That mission doesn't specifically require Jyn. My gear 8 Chirrut, K2, Cassian, Bistan, and Pao managed to get 3/6 on auto, so I'm sure a more geared team with Baze or Pathfinder could 6/6 it without a leader.

They couldn't back when I was doing hoth, of course back then there was no such thing as relics or even right-side g12. P5 & P6 can be pretty brutal without Jyn's protection regen and stun. (The stun really helps defeat the counter-attacking tanks.) Once I dropped a zeta on Jyn I could clear 6/6 with everyone g11 and didn't have to waste precious g12 gear.

Nowadays, would I spend the zeta on her? I don't know. I don't really regret it. It's actually great for GAC at earlier levels on defense. And I've got 170 zeta mats sitting around waiting for me to figure out a good use for them, so as hard as it is to collect zetas early on, it does get better and you eventually reach an equilibrium.