Replacing Structural Post

I enjoy posing problems for our community to solve. Sometime, these could be helpful object lessons ...

Large yard re-do, including new high end wood deck, pavers, grass, bbq area, new landscape irrigation ... you get the idea.

Deck is partly under a covering deck on the 2nd story. Structural posts support the deck, 2nd story deck and beefy roof framing (8x8) beams which hold a heavy traditional Spanish tile roof. Post are set in footings with typical saddles.

Ribbon joist connects to the outside of the posts in the front. Original construction was unbelievably bad ... so I inspected the post-to-footing connections for signs of rot. The pictures below show you what I found.

PROBLEM: The termite and rot at the base of the leftmost post runs upward in the 6x6 about 18"-20". Honestly, except for the stucco, I don't know what is keeping that post functional. Of course, it is the CORNER post so the 2nd floor deck and roof are supported by it. Fortunately, all other support posts are solid with no rot.

How would you repair this mess (I already have a plan) to restore a 6x6? Pre-condition, replacing the entire length of the 6x6 is not allowable.

(1) How do you stabilize the support right now?
(2) How do you replace the rotted member with new wood?

If I was asked, I'd recommend a Sonotube well below the frost line, and extend it to beyond the rot in the wood. Obviously, this old post would have to be removed temporarily and the structure above it must be supported temporarily.

In Southern California, the only frost we get is on the craft beer glass.

My plan re-uses the existing footing which is sound. Hope other pros toss some thoughts into the mix.

If you can't replace the entire post you'll need to either sister another post onto it and leave it, which I wouldn't do, or cut off the damaged plus some and replace it with the same size lumber, using either a lap joint or better yet, a double scarf joint.
You'll need to support it on a temporary basis. I doubt that it actually supports the roof, though it may be connected to the roof. I'm assuming that it's a 16 or 18 foot tall post.
You could sister on another 8 or 10 foot post and bolt it to the existing. let it sit on the existing footing or enlarge it if there isn't enough room for both posts. Cut off the damaged and create either a lap joint or double scarf joint. Use bolts on the joints.