To do it right would be to replace the case. Short of that, as a field repair / MacGuyver fix that may be worth a shot...
It looks like you have a contiguous sealing surface along the red line. If that plug is sealed, you may be able to leave bolt "B" out entirely or snug it up after all others are torqued - beinging careful not to distort the pan and put undo squeeze on the redline sealing surface. After that, provided there are no points of entry for debris where that plug is you could leave it like that or fill up that area inside with something solid - just as filler and then slather some silicone around the edge to keep dirt and water out.
Again, this is by no means optimal. 
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'99 996 Arctic silver over black. In the past I've owned and wrenched on a '96 Audi A4, '86 944 NA. '79 928, '72 914. Non-Teutonic vehicles of interest: 3 MGB's (one converted to V8 power), Triumph Stag & TR250, Jensen-Healey, T-Bird SC. Most recent "project" was a Factory Five MKII Roadster.
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