Did the recipe you saw call for a lemon cake mix, lime jello and key lime juice? I made one using those ingredients, but it turned out almost wet! 
It tasted really great, but just had too much liquid in it. It wasn't underbaked, it actually was cooked all the way through, but it was on the verge of being soggy because of the amount of water added (due to the jello) and then the lime juice. Next time I make it I'll reduce the amount of water by at least 1/3...maybe even more...lol.
As for your filling/frosting question....I made a whipped cream cheese icing and added key lime juice and zest to it....really delicious! I used it for both the filling and the frosting. But you could make a lime curd for the filling, then the whipped cream cheese frosting to ice with. Or - 
- You could make the whipped icing with the zest and juice for the filling and just the plain whipped crm. cheese icing for the outside?
If you can't find Key Limes in your area this time of year, they sell a bottled key lime juice...I think it's Nellie & Joe's brand, or something like that...it's made in Key West, FL (or South FL at least), actually. Very good. I try to keep a bottle of it around during the off season. And then use the zest from regular limes. Hope this helps! 
Key lime is my favorite! I have a key lime cake recipe that also uses lime jello. Then, when it's done, you poke holes in the top and pour a lime juice and sugar mixture over it. Might be the one mentioned above, but I've never thought of it as too moist. It's on the firm side. It's a scratch cake but is done in one step. If that sounds right, I'll be happy to post the recipe.
Key lime is my favorite! I have a key lime cake recipe that also uses lime jello. Then, when it's done, you poke holes in the top and pour a lime juice and sugar mixture over it. Might be the one mentioned above, but I've never thought of it as too moist. It's on the firm side. It's a scratch cake but is done in one step. If that sounds right, I'll be happy to post the recipe.
I've seen recipes for the one you're referring to, but the one I made wasn't like that...no poking involved...lol. I do love a good 'poke cake', though! This one was just a cake mix, prepped as directed on the box, but add a box of lime jello, some more water and some lime juice. The flavor was really great, but it was really almost wet. I don't think there's such a thing as 'too moist' but it's different when it's almost wet...does that make sense? The way I knew it wasn't undercooked was because it wasn't doughy at all and it wasn't wet just in the center of the cake...it was throughout. You could tell from the crumb that it was a done cake batter, but it was just very, very wet looking and sounding...lol. I was thinking about this earlier and thought maybe I'd just leave out the extra water entirely and just add the lime juice. With the water called for on the mix plus the lime juice, that should be plenty.
dtmc - glad my suggestions helped! Let us know how it turns out 
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