I have a couple of questions to ask about your panel first.  Is it going to be installed into an opening in a wall or door?  Is it going to be free hanging in front of an existing window?  Is it on an exterior wall exposed to the elements?  These are the various factors that have to be considered before deciding on the best construction method and reinforcing.  Copper foil windows are usually quite stiff and work well for interior panels.  The size of the panel that you are working on is not too large that the reinforcing strips should work fine.  The exception would be if your panel were in a door that was used a lot, especially an exterior door then you would want more substantial reinforcing.  Generally for reinforcing you would run the strips the shortest distance; about every 18".  You certainly could reinforce your panel a couple of times, at 10" and 20", without it being overkill.  Let us know if you have more questions.  Good luck. 

It is easier to cement the panel first and then attach the rebar.  If you are concerned about stability while you are turning it over to cement you could attach the rebar first.  It is ok to attach rebar both horizontally and vertically on a large piece.  In fact it is a good idea to form a "T" towards the bottom half of a panel with horizontal and vertical rebars.  Good luck.

It helps to let the marker dry a little bit before applying the Mark Stay.  It is more important to let it dry if you are using a paint-type marker.  I usually mark my glass and then let it dry for about 20 minutes before applying a liberal amount of Mark Stay.  Waiting 20 minutes for the marker to dry is a lot like watching paint dry so I will draw the patterns on a variety of pieces and while they are drying I cut the easy pieces that do not require the saw.  This helps with water based markers also but sometimes it will just lift off with the water.  I generally have better success with the paint markers.