6x6 Cedar post rot

I need help in repairing or replacing one (or more) rotting cedar posts that were improperly installed 9 years ago when my house was built. Here is the setup...

I have a screened in porch approx 16' x 12' that is 9' above a concrete patio. The porch/deck is attached to the back of the house (16') and to the side of the house (8') where the house wraps around the side of the porch. The porch/deck extends about 4' beyond where it attaches to the side of the house and then it has a small platform at deck height with three foot wide stairs going down from right to left. I will post pictures when I can but it looks something like this from above. I don't have access to take and upload pictures right now so I will have to opt for the thousand words.

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house

Construction is cedar for posts and decking, PT for substructure. There are three equally spaced 20' tall 6x6 rough cedar posts supporting the outer deck beam that extend from ground level up to support the porch roof. They are notched at deck height and the deck beam is inset in the notch. There are also four 6x6 cedar posts that support the stair platform and the outer stair stringer and stair rail. They are spaced with two at the plaform corners, one at the middle of the outer stair stringer and one at the bottom of the outer stringer. the inner stair stringer is bolted to the 20' tall posts and there is a middle stair stringer.

I was inspecting the posts on Friday and found that one of them has severely rotted and has now become home for a colony of of carpenter ants. Thankfully it isn't one of the 20' posts but is a 14' post that supports the stair platform. With some probing I've determined that the rot and decay now extends up about 2 feet above grade! After digging around the post I found that the posts were embedded about 3 feet deep in concrete and then backfilled with dirt. The square concrete form extended about 2 inches above grade but had cracked in a few places (appears maybe set with dry fill of sakrete and allowed ground moisture to set it up). Much of the below grade concrete easily broke away from the post and I found soggy wood behind it. It seems like the concrete probably held moisture in and allowed the rot to occur and the post began to rot from the inside out.

None of the other posts exhibit external signs of rot or infestation but because they have the same square concrete tops just above grade I assume they also extend 3 feet or so down and are fully embedded in concrete. I expect them to be susceptible to same rot problem. A couple of the concrete tops show similar cracking.

First, I plan to cut off the bottom of the rotted 6x6 post after supporting the stair platform with an appropriate support. I will dig out all of the old wood post and concrete that is underground and pour a new pier in a sonotube to a few inches above grade level and will use a Simpson standoff post support anchored in the concrete. I intend to dig and inspect all of the other support posts after completing this one though one of them has patio poured around it and I can't dig around it.

My questions:
1. Is it possible, feasible, or safe to splice in a piece of 6x6 to replace the rotted piece using a lap joint and carriage bolts? Or, do I have to replace the entire 14' 6x6 post? Or, alternatively, chould I just extend my concrete pier up two feet above grade level (not my asthetic choice but I'd like input on feasibility since 14' 6x6 rough cedar is about $100).

2. Based on what I've found, is it best to go ahead and cut all of the other posts above grade and pour new piers with metal standoff supports? I am thinking I need to be proactive on these repairs even though it will be a lot of work to get to a couple of them. I am concerned about the structural integrity of the 20' posts that support the porch and roof that are integrated into my house!

3. Assuming answer to #2 is 'yes', is there a metal post support with standoff that is well suited to retrofit since I won't be able to move or remove my posts, e.g. I will have to pour under and around the existing posts.

4. Anything I missed or that I should be thinking about?

Thanks in advance for help, suggestions, and advice.