I was the one who brought that up on A51, and the simple answer is "yes, it's on-topic here." However, in practice, this site has only accumulated 13 questions under the off-road tag in a couple of years. And none of those are about avoiding stumps or dealing with loose terrain or posture when doing downhill runs. So we haven't done much to encourage off-road questions.
Looking at your example questions, they're almost all clearly about motorized ATVs. So perhaps there's no reason to exclude bicycles from your scope, but simply let people know that this site exists. Similarly, we might encourage questions about trail-building to go to your site. (I have a feeling you may be covering those.)
There have been bicycle questions on other sites, like SkepticsSkeptics and Jewish Life & Learning, and that's a good thing. So while off-road is clearly on-topic here on Bicycles, perhaps a dedicated off-roading proposal can encourage more questions about these topics. A little overlap isn't a problem, as long as people are clear on where to ask a question. You'll need to figure out how to present this in your FAQ, as will we, if and when your proposal becomes a site. Something like "Off-road, focusing on the issues faced off-road vehicles. While we cover off-road bicycles, questions about mechanical issues or general bicycle riding might get more answers at Bicycles." But who knows? Perhaps your site will attract a new audience of off-roaders. That would be great for your site and ours.