That's extremely annoying, I always assumed that the oversteer was primarily a consequence of lower traction on low-g planets combined with the SRV's high centre of gravity, but if it's an issue where they actually have it performing as a RWD vehicle that's the worst possible setup I can think of for an ATV. It's one thing to force us to adjust to driving on planets with low gravity and rough terrain but to make the job harder by giving us a vehicle completely unsuited to off-road tasks without proper AWD is another issue entirely. If anything an electric vehicle with independent power to each wheel plus the massive low-end torque curve (essentially 100% of maximum torque is available from a standing stop) should make it an ideal off-road vehicle if it had a proper traction control and AWD system. It seems like one of those issues that FD has made harder than necessary for a "challenge". At this point I've gotten used to driving the SRV so that I can handle it fairly well but it's rather disappointing if they "nerfed" the design on purpose.
I'm sure that everything you mentioned is also a factor, especially the centre of gravity and low gravity affecting the handling. I'm as sure as I can be that the drive bias is at least strongly towards the middle and rear wheels with no drive to the front though. There's one classic test for RWD that seems to suggest it is:
Go somewhere with a steepish crater or mountain on a moderate G world, say anything from 0.5 to 1.0. Drive up the crater wall/mountain in a straight line until you find a bit where you just about come to a stop spinning your rear wheels, but that looks like you could actually get up it (i.e. not a vertical wall.) Do a 180 and go up it in reverse. I bet you get further up.
Back to modified embedded firmware, had to post this. Funny thing is I wasn't specifically looking for it, I was actually hoping to snag some specialised legacy firmware because something weird seems to be going on as far as degraded/encoded emissions around earthlikes is concerned and I haven't found a satellite for ages. I'll take this though, I don't need them right now but I will at some point: