There's still 10 days left for Koko, so you can still try whatever single pulls you manage to save up. People have gotten 5* in a single pull before, it's all down to luck (of which I wish you a lot of!).

Yelan is a really cool sidegrade to Xingqiu and they work amazingly well when paired up. But wether she's worth it for you depends on what you want from that new hydro and your personal tastes. Hydro Archon still could be anything role-wise, so it's not possible to say wether they'll be better than Yelan for whatever you want or not.