See screenshot in Husky's reply:
https://www.qcad.org/rsforum/viewtopic. ... 615#p38817
The second last option in the Option Toolbar when pasting, the one before the 'Restore defaults' icon.
If you don't overwrite block's then QCAD will use the existing block definition.
In step 4 you change the unit of drawing C and delete the Block Reference ... But the block and its definition remains
In step 6 you delete the existing Block and the underlying block definition, all related Block References will be removed too.
Your problems start with Altium Designer not exporting in mm.
Please attach such a so said mm drawing so we can verify why QCAD considers it to be in inches.
Regards,
CVH