Doug has summed it up ! Soft solder is fine for non load bearing applications such as electrical joints or copper tubes but totally unsuitable for fastening an new flag to a key.

If you look at a new off-the - shelf 3-lever mortise lock key the flag is electrically welded to the shaft an thus stays put.

In order to silver one needs to get the parent materials (flag and shaft) to say 370 degrees F. Soft solder melts at under 200 degrees F