You have asked a lot of questions...good questions tho.
Can you read a geometry chart? Compare stack and reach on the Roubaix to the Crux. Post charts if you want here. FWIW a Crux has 30mm or so longer fork crown to axle to fit taller + wider rubber but has a much shorter head tube...different fork rake and slightly less BB drop aka higher BB.
There is about a one size equivalency difference. A bit odd but that is how the convention turns out if you crunch the nos. A 58 Roubaix has almost identical stack and reach to a 61 Crux.

Now...the difference in the bikes. It all depends on the ratio of dirt to pavement you ride. If you spend 80% of the time on pavement, get the Roubaix. If you need to, you can run 28c tires on a Roubaix. Its a faster bike than the Crux on pavement, climbing and smooth dirt. If you are going to ride rougher dirt, mostly single track or gravel, get the Crux.

Btw, the Roubaix is far from plush. My SL3 is as stiff as the Look roadbike it replaced with even better power transfer. The SL4 is even closer to the Tarmac in stiffness.

Either bike is great. A Roubaix is a very versatile road bike and a carbon Crux is a very nice carbon cross bike that can handle the rough stuff even better but a penalty on the smooth stuff.