Joe,
You can use your TMCC system as a back up until your Legacy system is repaired. Lionel does take special care repairing Legacy systems, they did my train clubs system and were very quick about it. Things may of changed as that was several years ago. The TMCC system will run all you Legacy engines, you should not have to pre-program the engines just enter the engine # on the Cab1 and run. You can’t do a lot of the Legacy options, no quilling horn or whistle but they will run until Lionel returns your Legacy system. I am currently running a Legacy 2 engine train on our clubs traveling layout that only has TMCC. The hardest thing was programming the Train using TMCC but I googled the TMCC operating manual as I forgot some of the TMCC steps in setting up a lash up.
JohnB