Steel rims responsible for slicing tubes?
OK if the tire was blown off the rim then I highly suspect that is the whole story.
When the tube leaks out the because tire bead has come out the tube will burst and you will see a slit.   

A tube outside a tire cannot take much pressure.  

Most likely when you went from 60 to 85 psi it was enough pressure to push the tire off the rim (bead).  

I have seen this with some older bikes.  The rim is a little small.
Look for tires that run small and run at lower pressures.  

You might also have a mismatch like a 27 on a 700c.

A Schwalbe Marathon Plus is a pretty hefty tire and tend to fit a little tight so this is kind of strange.  But it did happen after you went to 85 PSI. 
Maybe give it a try again at 60 PSI. Even if you get a new bike you would have this as a backup.