Window Sensor - need advise (w/ pictures)

Hello,
our house progresses and now we have the windows in. I checked out where I want to run the cables and looked at where to place the contacts. I purchased the standard press fit hidden magnetic contacts. I noticed now that the windows are set further out than I thought due to the stones below. If I drill them I would end up inside the OSB wall board and not through the stud. I investigated if I could angle it but I think this would mess up the sensor placement. See pictures:
 

 
Its 3 1/4" from the edge to the wall.

 
Its 3 5/8" from the edge (some are 3 3/4") to the center hole from the window. 

 
Should I go with surface mounted contacts. Any thoughts?

I'd place the contact in the widow track where you want it and angle the wire hole through the stud to get there.
The contact itself is only around 1/2" deep.
As long as you have access from the inside of the wall this should be doable.

Like this? I am afraid that I am too much in the OSB.
 

I'd go surface personally. We went with them (mounted around mid-height of the window, basically near the top of the moving portion) in our home. Once we put up blinds/window treatments, they might as well be invisible - we never see them.

Like This
 

Due to all the bizarre funky window styles and each with there own warranty requirements I now go wireless on all windows. I like the Honeywell 5811 transmitter. If you are going the HW route I would use a 3/4" hole and adapter for the 3/8" contact. This should give you enough room to angle the drill bit and hit the stud cavity. I have also found the DSC secure wireless vanishing transmitter to be a disaster. The sunlight deteriorates the case to the point that the plastic breaks during the first battery replacement if you are not real careful.

I go surface, I found a sticker on all windows that said all the warranty is void as soon as I drill a hole in them.
 
So the bottom of the moving pane has a 0.59" x 0.59" area I can glue a big magnet into. I tested it today with a contact that I had around. I can pick up the magnetic field through the aluminium pane. If this works I would only see the contact and not the magnet. Now I am looking for the best magnets that I can use. Do I need pairs or can I mix any magnets in?

Any recommendations?

Those look like aluminum extrusions....if so, generally you can drill the sill at an angle and still get inside the bay easily. Warranty is typically on the thermalpane section only, but check with your manufacturer.
 
Conservative is surface mounted, but you will need to contend with the extension jambs and whatever finish material is being mounted on them at some point.
 
There's more room there than you think, especially if you go with stubby recessed contacts (3/4" OAL).