The potassium permanganate bleach makes the emulsion extremely soft and fragile. I have seen the emulsion essentially melt away in the second developer as well. A couple of suggestions: Use Kodak D-19 with the thiosulfate for your first developer and unaltered D-19 for the second developer. Keep temps near 68F. Do not let the developer get too hot. Also, you may want to cut back slightly in the amount of potassium permanganate used to make the Solution A bleach. If reversing an old-school emulsion like Efke/Adox the permaganate should be roughly cut in half (from 2g to 1g per 500 ml water). Do not alter the strength of Solution B. Do not worry about "over exposure" prior to second development. Bring it out in the light and thoroughly expose the emulsion taking care not to scratch it. Always use a hardening fixer after second development.