If I recall correctly, gelatin melts because there's a huge shift in pH between the first developer and the bleach. It's not because of the permanganate. It does nothing to harden the emulsion, contrary to what dichromate does. So keeping that shift to a minimum will solve your problem of gelatin melting.
Also using a temperature of 18°C helps.
Try using the first developer with a diluition factor like 1+3 or 1+4 and lengthen the developing time accordingly . This will slightly lower the pH. Then try to cut the permanganate in half. The idea is to find the bare minimum of each and every chemical specimen that will work one-shot and will avoid ruining the emulsion.
Try also to add some glutaraldehyde (such as Rollei Rbm5) in a bath between the first developer and the bleach, sort like of an hardening stop bath.