Put your fears to rest.  There are more than one safety device designed into the receiver.  First the single lug as you are aware.  Second the bolt handle slips down a slot in the receiver.  The bolts on the M1903s, M1917s, and mauser types are behind the receivers and have a different safety scheme.  Thirdly, I seem to remember a locking slot milled into the bottom of the bolt itself.  I don't have my books here in Africa so the smarter guys will have to chime in on this.

Your concern of running low powered loads means you will not be hot rodding the .30-40 round.  The Ordnance Dept tried that in the late 1890s and found the hotter round was cracking metal of the single lug.  However cracking is not catstrophic failure.

Which brings me to the next point.  Double charges of fast burning pistol or shotgun powders will cause a failure in a very bad way.  Be careful on your reloads.

I own sn 112.  I have fired factory R-P and Winchester 180s in it with no problems or fears.  I have also fired my own reloads of sierra RN 220s and forty grains of IMR 4350 through it and still have one eye and six fingers.  Just kidding.  Still have everything I was born with at the moment.

Search the older threads in this sub forum and the archive forum for discussions on reduced loads.  You can jump on over to Culver's shooting pages' krag forum for reduced loads too.  Most of the guys here hang out there.  Hey.  A krag is a krag.

Best regards,