My opinion is that a stop leak product can possibly seal a head gasket that is leaking externally or possibly between a coolant passage and an engine oil passage.
Stopping leaks into the combustion chamber is a different kettle of fish and pretty much a waste of time but based on what you’ve stated I guess no harm, no foul. You’re not going to hurt it at this point.
Off the top of my head, do the intake manifold gaskets have coolant cross-over passages? If so, maybe the problem is nothing more than a leaking intake gasket which is allowing coolant to be drawn into the cylinders.