So, I did not know about Broken Oar Grotto having a glitch until it is way too late.
I love having Cicero as one of my followers, he is so fun, and being unkillable doesn't hurt either. Needless to say this is how it went down. We finish the mission & leave and he is not with me. I go back in and out a few times & nothing. Not knowing the severity of the situation, I figure I will dismiss him inside & he will make his way back to the sanctuary eventually (wrong). I continue on adventuring. Go back to the sanctuary a few in game days later & whoops nothing. Hmmm. getting concerned now & pissed. I go back to the Grotto & he is still in there.
Now I try some wacky things. I try Fus Do Ra'ing him towards the exit, nothing. follow me again, nothing. Now I think maybe if I go home & get the wabbajack, I can turn him into a sweet roll & carry him out, nothing. It will not change him, it just does shock & stuff. I wipe out almost a full charge of the staff. So know I figure maybe I will try fighting him normal & he will attack & follow me. I forgot I gave him a sword that has a percentage chance to paralize. So almost everytime he hits me I fall over like a tree. Next I am dead & it reloads him attacking me. 2-3 deaths & reloads later I finally run out before dieing & NOTHING. svcks... Oh wait. NOW Shadowmere is pissed of that I was attacking Cicero inside so he is knocking the hell outta me with his hooves. So much fun.... Reload to my file right before going back to him again. Sad, he will have to stay unless he is forced out after a 30 day respawn (hopefully) or they patch the glitch.
Sorry so long. I needed to vent my tale of WOE.
poor Cicero

I don't like Cicero, so to me that would be the ultimate relief

But I'm sorry it glitched =/ I don't know of a way to fix it, sorry =/

I had know idea Cicero could be a follower. That;s kinda cool.

Seeing as it's Cicero I consider it more of a blessing than a glitch.

Wow. That's F-ed. I'm wondering what the standard respawn of the dungeon will do too. Hopefully I'll remember to dismiss any followers I have if I ever go back there.

Interesting what you said about the Wabbajack. I had a similar problem trying to reset Mjoll's level to max. I had previously done it with Lydia, but when I tried it on Mjoll (several game months later) I only got the offensive fire- and ice-based 'spells'. I tried it in Whiterun and in Riften. I tried zoning in and out (Riften), exiting, relogging, zoning again, but kept getting the offensive effects. And, before peolple ask/speculate: yes, she was dismissed before I tried it - it's why I didn't zone in/out in Whiterun because I figured she'd be gone when I zoned back in. FWIW: I think her level was probably already maxed because I'm pretty sure I was past 40 when I went to Rifen to start TG stuff.

Think I might be able to help you if you are using PC? Wich is a good thing to mention if you want help.

Try using the console outside or away from the grotto, write this: player.placeatme 000550F0

If you're not on pc I have no idea how to help you sorry.