Many of the points raised have already been answered, but I want to relate my own experience. I am a mainly road cyclist, with occasional (summer evacuation) mtb trails.
Until today I have never transport a racing bike by car (always starting from home) but I would like to be able to do so to look for new routes.
On my Defender 90 the two mtb were placed in a rack mounted on the spare wheel (very nice, square tubes, aluminum and support arms), but it has always been a solution that I didn't like. 600 km of highway hanging outside is not the best. Dogma F.
Acceptable for mtb bikes, absolutely unthinkable for a 15,000 euro Pinarello
One of the reasons why I switched from the Defender 90 to the Grenadier was the possibility of transporting the bikes inside, even if, I must admit, the impossibility of folding the seats and obtaining a single level floor (the biggest flaw, for me, of the Grenadier).
However, the possibility indicated by previous posts remains, to load the bike without front wheel and fix them with one of the supports indicated above (there are dozens of types).
The question remains whether to place the bike with the rear wheel facing forward or vice versa. With the front wheel in front, it should not be necessary to lower the saddle. I believe that the support can be supported without problems from the bottom of the seat, also because the bike weighs 6 kg without the front wheel ... What do you think?