OK, I finally got that 2x4 into the mast sleeve with some effort (it worked, but I will not be able to resell the sail in "as new condition" anymore).

There are some benefits though (even with the few rips in the mast sleeve). After nailing two 2x4s together and jamming them into the sail, I can get that perfect length mast buy just cutting of the excess with skill saw, no extension required. I guess with that perfect mast length, I don't need to worry about the DIN / IMCS / MCS number any more.