1. There's not automatic way to create a tempo map from Markers, but this goes pretty quickly:
- Set the Now time at each Marker (I have Previous/Next bound to Ctlr+PgUp/Down) in turn.
- Hit Shift+M to open the Set Measure/Beat At Now dialog, enter the measure and beat 1, and OK.
- If the tempo is relatively relatively steady, and just drifting around a little, SONAR will probably guess the correct measure and beat in most cases after the first, and you can just hit Enter.
2. Inserting user transient markers is also a manual thing, but it's going to be problematic because even with all the detected markers disabled, SONAR is going to enforce rules about how far markers can be moved, and not let one marker move across another. Better to just snap the nearest existing marker (dragging by the center diamond so you don't stretch audio) to each measure after you've gone through the above process, and then disable everything else. But If you've already created a tempo map, I'm not sure why you would need matching transient markers...?
3. You can split at transient markers if you've done the above, but it would be easier to split at measures based on the tempo-mapped timeline.
4. I don't think you can hide markers.