You poor thing! I don't have any sage words of advice, just wanted to offer some sympathy. I'm just starting and havent had such a big cake to do. Makes me want to just put cakes together at the event instead of trying to haul them in one piece. I wonder the disadvantages and advantages to that? Chin up!
I made my first wedding cake last weekend. It was 4 tiers. I used smbc and it was hot, but not Texas hot. I used Wilton's round, separator plates between tiers...except for the top, 6 inch tier, I didn't have one. A friend and experienced wedding cake designer had suggested using them. I used wooden dowels and then pushed the legs of the plates into the top of each cake. This helped prevent sliding. The cake was delivered an hour away via winding, bumpy, back roads. I put the fresh flowers on top, when I got there. We took the middle seat out of the van and I sat there with it, just in case, until I got sick. I watched it sway a little, but never shift. I think the separator plates really helped.
Oh, poor girl. I know, first hand, how horrible this is. I wouldn't wish it on anyone. You can travel with stacked cakes, but there are lots of precautions to take. I know I probably over do it now; but this is one of those occurrences that you only allow to happen once in your life.
Don't feel bad, it can happen to anyone. Cake deliveries are ALWAYS a nightmare. I always take a picture of the cake when I am done.....Just in case. And call me anal, but I have NEVER traveled with a 2 or 3 tired cake together. I have always gone early to the venue and set it up. I did transport my last baby shower cake assembled cause I was SOooo late leaving. But I only had to drive about 20 km.
But with heat & humidity, still, sometimes it is a battle. There is a lady from Georgia on this site who gave me her recipe for buttercream that withstands heat & humidity. I know it isn'y as hot here in Ontario, Canada as Texas, but we do have wicked. humidity.
Again, don't beat yourself up. I am sure your friends knew just how badly you felt.
From one Zurek cake decorator, keep your chin up and chalk it up as an expierience you won't want to repeat. Good luck on the next one it will turn out fine!!!