Now that is a unique bike.

Bilenky semi-recumbent tandem

And it helps one to better understand the point of the project.

If it is a low volume mostly custom build, it may well have straight tubing which will simplify things a bit.

Do you have access to new chromoly tubing for splices & etc?

The idea of butt joint welds is probably ok, although alignment and mitering will be critical. Another option is to add an internal sleeve, and braze.

A couple of things I'd suggest. First would be to take the owner's favorite bike, and see what it will take to match the seat position and the handlebar position with respect to the bottom bracket. That may be simply moving the seat forward/back on the seat post and getting a different stem, or it could mean some radical revisions.

Is that a water bottle cage mount on the head tube? One on each side? A most unique place to put it, but it does get it out of your way. They are also commonly put on the seat tubes.

Oddly, your chainstays and seatstays appear to be nearly the same size. That could mean the seatstays carry more weight than other bikes. Looking at the seatstays. It appears as if you have some kind of hydraulic rim brake. That means you have to more or less maintain the distance between the rear dropout and the brake bridge, which brings you back to rebuilding your seat cluster. Doing so may not be as bad as it looks, although you may need to ream the seat tube afterwards. Another alternative might be to use bent seat stays, and change to disc brakes, but that is a different project to make the mounts.

Fortunately there is no need to worry about toe overlap, so you could shorten both the top tube and bottom/down tube if necessary.

Whatever you do, I'd build it so it would take a mighty long seat post. I'm seeing some about 410mm long, 27.2mm diameter seatposts. And you need to provide room to drop it down 100% to the bottom, or raise it as high as it will go (thus giving flexibility for rider configuration).