freewheel stress and possible difficult removal with super underdrive gear? I tried to research this online but didn't see any examples of this exact concern, which is if I gear down a very cheap mountain bike that has a 7 speed freewheel, do I run the risk of creating so much torque magnification that something bad could happen?  Is it possible that something could break from the high torque?  What about if and when I need to remove the freewheel?  Might it be stuck from the high torque pedaling in the lowest gear?

Stock lowest gear is 24/28 and I will attempt either 20/28, 24/34, or 20/34 which is about 15.3 gear inches.  I ride mostly on the street at a slow pace (about 10 MPH average) but I also like to ride thru grass slowly and even in grass uphills so I put a good deal of force on the pedals in that situation.
Yes - chains can be broken and will also wear quicker when under stress from low gearing.

I have snapped a almost new KMC chain while on a 15% climb.   I would have been in 26/42 on a 26" MTB.  Fortunately I was going very slow.  The pedals spun, so I clamped on brakes while unclipping.  The slope meant I couldn't reach the ground straight below the seat, and so the whole bike and I fell over to the left (our footpath side) and tried to slide away down the hill.  Was quite embarrassing, but fortunately not painful or damaging.  One of the chain's side plates had come off the pin, so it was only a matter of time before it wrenched the other side out completely.  Never buying another KMC chain!

Removing the freewheel can be hard, so remember to apply copperslip or anti-seize or even plain old marine grade grease to the threads before assembly.  The leverage of the rim means you can exert a lot of force when undoing the freewheel, provided the nut is held firmly in a bench vise (and said bench vise is fastened to a bench!) 

Freewheels tend to seize their threads, but sufficient power can undo them.  Cassettes can "erode" their way onto a freehub body, and be unrecoverable completely.

Finally never underestimate the torque advantage of a handy piece of pipe.