I don't think I'd swap many of your tri components into the MTB.

One option on your tri bike is to part it out and sell off the components.

I'm not sure about finding lightweight MTB frames. Suspension? No suspension?

There are quite a few 10 to 20 year old aluminum frames that you should be able to snag cheaply. Also, perhaps consider direct import Chinese carbon fiber (E-Bay, Alibaba, AliExpress, etc).

Also consider some of the high-end Hybrids that are now coming out, or better cyclocross bikes.