Question 1) Does anyone know if the secondary mirrors in the SW Heritage 130P are protected?
A) Not sure but would be very surprised if they weren't
Question 2) Should I clean it?
A) Probably not unless it get really dirty. With secondarys, a bit of a soak in lukewarm water with just a drop or 2 of dishwasher detergent, then use some cotton balls to run over the mirror letting gravity moved them (ie not scrubbing) then rinse in demineralised water and sit vertical so the water runs off.
Question 3) If I stuff up, are replacement mirrors available and at what cost?
A) Almost certainly not available as a specific Skywatcher part. You might try shopping online for a similar size secondary. The mirrors on these are 34.5mm on the minor axis which is roughly 1.36". Astro Systems sell a 1.3" mirror for US$50 plus freight (see
http://www.astrosystems.biz/secm1.htm) but you would need to see how you go about fitting it.
Cheers
Malcolm