I can't completely agree your final list

Good! A visit here without learning anything but bloody recipes will be a dull day. And yes, sweetie-darling-sweetie, once an actor, always an actor, I sur-pose

Yep: the repetition of the ‘s’ sound was a simple blooper of cut’n’pasting – shorry.

You say:
"Mission" and "Tension" are identical usages. The double "s" in the first is a function of the preceding vowel, which should have its pronounciation altered if there were only one "s". The bit that gives the "SH” sound is the "si-".

Leaving pronouns aside, is this true? What about the reason for why we pronounce missing completely differently to mission? Wouldn’t your suggestion imply a pronunciation more like “mis-shon” or even “mis-she-on”?

Tell me more.