I've realised that I missed a Stone of Barenziah while I was in the Thalmor Embassy during the quest Diplomatic Immunity.

I understand that there are three ways of getting back in:

(i) By jumping to the right spot in the Reeking Cave and using Whirlwind Sprint to get back up to where you came from.
(ii) By using a horse to jump over the fence.
(iii) By using the console.

I generally try to avoid using the console, but since (ii) is clearly an exploit, it seems like I might as well console my way in. (i) seems fractionally less like an exploit (because Whirlwind Sprint is supposed to be able to get you to places you couldn't otherwise go), but given the fiddly nature of the jump, it does seem unlikely that Bethseda intended this to be a way back into the Embassy.

So should I (a) use Whirlwind Sprint to get back in (in which case I can at least pretend it happened in character) or (b) use the console to get back in, since it's clearly ridiculous that I can't get the last Stone or ? just accept that I've missed my chance and I'm doomed to be carrying all this junk in my inventory for the rest of my life?

(Also, how do I stop this editor from converting a 'c' in brackets to a ©?)

Just console it. Just a game.

you're putting too much thought into, pony over the fence and call it a day. those little stones do nothing but hog inventory space and weigh you down. its going to cost you more enjoyment having to forever carry around 20 little undroppable quest items than it is to use a magical gravity defying horse just once.

Try (c ) ....

The gem has been moved to Reeking Cave under the embassy if you previously completed Diplomatic Immunity and did the 1.4 patch. Find the entrance to the cave in the mountains west of the embassy and the gem is lying alongside a dead thief.

Horse jump is not an exploit.