The sand star must have been on its way out before the shrimp did anything, they don't kill
sea stars. They may have even been cleaning the dermal surface of the
sea star, as Pepermints often exhibit cleaning behavior.
Were I you, I would not replace the sandsifting seastar, as they have a tendency to eat the good bugs out of the substrate. If your water parameters are really good, and y9ou have rock steady salinities, then you could consider getting a
Linckia spp. sea star, but your salinities must be rock steady (as in you use some method of automatic topoff to maintain salinity.)
Fromia spp. would be good as well, not as sensitive to slinity as
Linckia are, but pretty close.
On the skimmer, break down and get a good skimmer or build a good venturi type skimmer. You'll thank me later.
Fixing the leak will depend on how bad the leak REALLY is. If you have a wet corner, then a fix is in the near future, but if it is just a little dry crust, then 3 months shouldnt be a problem. What is the tank made of?