It appears you have given this some great amount of thought.

I'm not home to go pull a plug and advise you on what next "up size" of thread to use that you can find a similar type plug to use.

The statement about a conical plug made me blink. (:). I thought you meant a cork or rubber plug (:).

I'm hoping you meant a tapered "dry seal" pipe plug thread like a 1/4" or 3/8" NPT.
Dry Seal is the design but a Teflon lubricant or various paste types are used to make them more sweat proof when it comes to diester fluid additives.

If you go that route try one with for hexagon or "Allen" style as they are usually made a harder steel.

When you use the tap put lots of grease on it to catch the chips.
Turn the tap into the hole only eleven full threads threads from the top side of the length of threads down.
This of course, is on a new unbroken tap or ground off the bottom tap.
This should give you a proper depth of taper.

As far as pressure there is none that I know of involved in a manual transmission.

Good luck

Phil