Palpatine event. Can I even unlock him?

Actually, 5* is totally viable for Arena or gw. Anyone that tells you otherwise is just wrong.

Next is yes you can prob earn him @ 5* but it's gonna take some grinding and maybe even some cash. I had 0 rebels even close to 5* at the start of last event & I unlocked him the last day after dropping about $250 to do so. I don't regret that in the least, he has been totally worth every penny.
There was a $50 bundle that you could only buy once last time, if it is available this time def grab that, it's a great deal. The rest cash went towards crystal refreshes, mostly energy but a cpl node refreshes the last cpl days. Also watch pve shipments for any shards of the char you're grinding. I unfortunately had to buy a bunch of the 1199 char packs which normally are no good but they were enough to get me to 5 5*.
Anyway, best of luck. He will be totally worth the time, money, and effort should you succeed. Even at just 5*.