Can I try addons using the WR1 or WR2 demo's?

No, both, MBWR and WR2 demos, do not show addons and you will not be able to play with addons in the demos.
MBWR and even WR2 are very cheap already, I guess there is no need to test the game with addons but buy it right away.


Bis denne, Han Solo

Is replacing the Original Cars with AddOn Cars working?

That would just replace the main car mesh, so all technical data would still be from the original car, and wheel positions won't fit, car would be too high (or low), driver would sit anywhere etc.
I think the official WR2 demo had just one car, the VW Golf (or a Skoda in the Skoda branded demo version), while WR2 full version features ~90 cars plus all the other tracks, rims and goodies plus you can add 1000 addon cars and rims, tracks ... I would say playing around with the demo is a waste of time, WR2 has become so cheap in the meantime ....