If it's a bicycle, it's on topic here. Doesn't matter where the riding is done.
All of the questions you ask are on-topic here, though "good off-road routes" might be a little tricky to ask in a constructive fashion.
Motorcycles are not on-topic here. (motorized bicycles with pedals and a motor probably are, though) Perhaps you should make sure off-road motorcycles (dirt bikes?) are clearly on-topic on offroad.SE.
Most off-road bicycling is called "mountain biking", though there's also "cyclocross" some kinds of BMX, and other types off off-road bicycling. If you hunt around we definitely have some questions in this area already. Most "suspension" questions are really about bicycles made for off-road, too.
You shouldn't worry too much about whether or not off-road bicycling is on-topic for us. It's ok to have sites with overlapping scopes.
Better questions for you to be asking: Do people that do off-road ATVs, off-road truck or off-road SUV stuff tend to often also do off-road bicycling? Are these overlapping connected communities? Are there likely people passionate about both? Do people riding bicycles off-road identify themselves as "off-roaders" in a way that would tend to bring them to your site? Does somebody who rides an ATV or drives a truck off-road have a good likelihood of having useful answers for somebody who wants to ride a bicycle off-road? Is riding wicked single track on a bicycle anything like off-road motorcycling?
Reading through the current example questions I don't see any questions that apply to bicycles at all. Maybe the tire question.