Mark, lotsa questions
Rollers on the back of the car to assist with loading would probably work well, If you're fit enough to kayak in the first place you should be able to lift any of the Hobies one end at a time. I have found picking them up lock stock and barrel to lift up and onto a car is an entirely different proposition - and I am not unfit - it is just that they are extremely cumbersome to manhandle and it is an even bigger challenge if you also have to turn them over when they are finally on the car - these are the reasons I invested in some cradles - they make cartopping the boats sooooooooo much easier.
Others have posted on ballasting the Oasis for single handing - the standard recommendation is bags of water or sand if you don't want to go to the extent of fabricating something more fancy out of e.g. lead. I have never ballasted mine because it is a 2013 model with front and aft cockpit steering so when single handing it I just jump into the front cockpit which puts your weight only slightly forward of the centre of the boat and it trims well enough with my 200-odd pounds there.
There are two problems with being in the front, though:
Firstly, you can't reach the rods in the rod holders from there (a problem if you're a fisho) - hence my observation that you can move between the cockpits reasonably easily (though a better solution would be to add a rod-holder or two for the front cockpit - shouldn't be too hard or costly);
Secondly, sailing the boat from the front cockpit would be tricky unless you installed another sail in the bow area - you wouldn't really be able to watch the sail to trim it if the sail were behind you!
If you add a foremast & sail and then tried to sail with that as a single sail from the front cockpit, I suspect that you would find that this configuration would only work on points of sail from a reach/broad reach to downwind because of the location of the sail in relation to the point of lateral resistance of the boat (the downwind pressure of a foresail on the bow would constantly be trying to push the bow downwind unless counterbalanced by an aft sail. Similarly a single sail at the stern would tend to make the boat naturally want to steer upwind all the time. Note that the standard mast base is pretty much in the centre of the boat so the boat tracks pretty well with just the standard sail.)
So, as you can probably deduce from all of the above, my answer to your question about the differences between sailing solo and 2-up is that, yes, I am sure it will trim differently depending on which sail (assuming 2) you used, where you sat and how the boat was ballasted.
Personally I am not in a hurry to try single-handed-sailing a two-sailed Oasis from the front cockpit! I have sailed my boat 'almost solo' from the rear cockpit with just the standard sail and some useless ballast (namely t'wife or 10-year-old daughter up front) and my friend's schooner with he and I on board; both made progress, but the latter was a really engaging little sailing boat with quite perky performance and all sorts of options for trimming the sails and keels (pedals) for upwind and downwind sailing, tacking and gybing etc. to keep the helmsman interested, plus the option of handing over trimming the foresail to the crew thus providing the helmsman with someone to curse at
The extra sail did not make the boat feel particularly tippy because of the presence of a lot of the aforementioned useless ballast, but this is said in the context of light-wind sailing only - you won't want to go out in anything strong. And the extra foresail did not make the boat difficult to helm at all - she still tracked well. Without a decent daggerboard you are not going to win any tacking battles in an Oasis, but if you could cobble together a couple of homemade ones (out of old windsurfer daggerboards for example) that would go in the drive wells instead of the drives, I reckon she'd be a proper little snake upwind!