Is it OK to shut off the A/C at the breaker

In the heat mode the larger copper pipe going to the AC unit outdoors should get very hot, if not then the refrigerant might be low or the reversing valve which switches modes from Cool to Heat may be bypassing. the fact it shuts down tells me its getting overheated likely due to low refrigerant level.

Interpretation:

Ok so, the air handler inside cuts out when it should be running. These symptoms became apparent once the house was basically air-sealed with insulation. And the unit is old.

Conjecture:

Insulation is the straw that broke the camel's back (it caused the fan to have to work harder to push the air). The blower motor is tripping-out on thermal overload because it's mostly burnt out; either the windings or the bearings (or both, now) are shot.

Solution:

Replacing the blower would probably get it up and running, but do you really want to drop ~$300 + labor to find out, on a 30yo unit? (Decide soon ...winter is coming.)

Confirmation:

Turn the AC off and set the fan to 'on'. If it kicks out every half hour: there's your problem right there (provided, it's not a lose wire somewhere, but if you can set your watch to it, I'd be leaning towards an over-heat).