A magazine I saw today featured a JIG for cutting Post Formed Countertops such as the one a reader had a problem with last week (and posted here).
Basically, it is two pieces of 3/4" MDF or plywood (one bout six inches longer than the counter top is deep and the other about six inches longer than the backsplash is high/tall.
These are joined at right angles so as to lay on the counter top (with the counter top positioned "right side" down).
A strip of wood is fixed to the edge(s) to guide a "skill saw" base plate as it cut through the MDF Support and the Counter-top below, then run "down" the back side to cut the back splash in the same fashion.
With the "good side" down, it appears to meet all the requirements for a clean cut and could be adapted ( I suspect) to cutting the 90's our OP required.
Sorry I forgot to note the name of the publication!