Yes, but... Why would you need to recast Shellra in less than 15 seconds anyway? It seems kind of pointless to me to buy Shell when Shellra can be cast on all members of a group for the same amount of MP. So it takes a little longer to cast... Big deal! If you're casting buffing spells like Shell and Protect, chances are it's not right in the middle of a battle.
yes shellra can be cast while soloing, just like protectra. but you won't be able to just cast it on someone without them being in your party. if you like to help people not in your party then you will want to buy shell.