I would suggest trying a different cake. Here's a link to some AWESOME variations of WASC. It's a sturdy, but moist cake, can be used under fondant & carved as well. http://docs.google.com/Doc?id=df4f9hbq_46cs9f28fs
Here's what works for me.
1) A thick dam---prevents that middle bulge. From Sugarshack I learned to mix in PS until thick (I like to be able to form a ball out of it)
2) After I pipe the dam I put a piece of wax paper on top of it, then a cake board & press down gently but firmly. I fill then put on top cake layer. I do the same w/ the wax paper/board again.
3) Crumb coat & let sit at least a few hours or overnight. If I don't have time for this I will sit a pan or board on top of the cake w/ a jar or can (not empty) to help w/ compression.
4) I don't usually put a lot of buttercream on the cake but will put a little more if I think it needs it. Smooth w/ Viva papertowel & pop in deep freezer for about 10 min. This hardens the buttercream, but not the cake. Done it w/ SMBC too.
5) Cover in fondant & let sit a few hours before decorating.
Is your dam firm? How much buttercream do you put on? The weight of the fondant may be squishing the buttercream when it warms up. Again try another cake recipe! I agree w/ the cakes being too cold. Also, you can freeze MMF, but it needs to be put in the fridge to thaw before room temp. You can put it straight into the fridge & then room temp w/ no problem. Another thing, once you've crumb coated you've sealed in the moisture & it will be ok at room temp about 5-7 days. Fillings are another story
Hope you're getting answers to your question!