no need to drill first.... if you strip a M14 the hole is 14 (or even larger), that probably is around the core diameter for a M15, if not M16.... plus it's aluminium, any decent thread cutter can deal with that.
sump will depend what engine you have....
mk4 sumps are aluminium, what exactly are you trying to get out with a magnet?
also flushing will not work since cutting thread will create quite a burr on inside.... holding back oil and debris