Suggestion: Include a readme into the blockchain_2015_07_xx.7z-backup that states something like:
When using this bootstrap, better do not recycle an existing MintCoin data folder, only keep your old wallet.dat.
plus some step-wise instructions like yours above.
This sentence exists already in the READ_ME.txt:
Delete all files from %APPDATA% EXEPT your wallet.dat !!!!!!
Is that it? If then sorry, I wasn't reading that file attentively enough.
... so again:
Perfect. That was it. Solved!
Thanks a lot.