Some answers for you
1) what is the size of the slider for a Z scale vehicle?
See attached photo - 4.7mm long x 3mm wide
2) can this system run up and down hills or does it need to be on a flat surface or at least a linear plane (consistent angle, etc)?
Yes it can be fairly flexible - like a snake but no sharp turns or kinks - given the strength of the magnets and weight of the vehicles they could possibly take a 45 degree angle from linear plane but it would be trial and error. Your base cover would have to be secured strongly so the track did not fall out if you started to angle the track into a banked curve but you might be able to get away with it in z Scale. Tell us how it goes!
3) I read that the tightest turns are roughly 3-4 inches due to the limitations of the track. Would it be possible to cut the track in places I would need a tighter turn and perhaps use a small dowel or wheel for the chain to curve around then enter the next section of track if the chain itself is capable of making tighter turns?
You are restricted by the chain link size of 21mm so the smallest curve would be the 180 degrees used in the return loop pack or included in the starter sets. You cannot defy the physics of the system regrettably by modifying other parts otherwise it loses its integrity. The links are made to take magnets and anything smaller negates that ability This gives you 60mm (2.35”) between centres of the two tracks - see pictures attached - at the turn - obviously you have a larger 86.2mm size to accommodate under the road surface. You can bring the two tracks in closer after the 180 but the closest would be 20mm between the track centres which would be impractical for any scale except Z as vehicles would hit when passing.
4) do you sell any accessories other than slides exclusively for Z scale layouts?
Not at this point in time but we are possibly the only one that offers an off the shelf system that gives some means of animating Z scale vehicles BUT you have to work within the limitations of the system.
Hope this all helps
Clyde has been involved for many years with the Magnorail system having built his own layout "Echo Lakes" featuring two concepts using the Magnorail System.