I drove a VW bug from the middle of Mexico to Denver on 3 cylinders once. The tip of a valve pulled off, the valve fell part way in, got smacked back and the stem bent, which kept the valve up high enough that it didn't hit. I pulled the rocker, finished the Carrera Panamericana race, and drove home from the border. I don't recall if I pulled the spark plug, but upon disassembly I didn't find any additional damage.
I once, at an autocross, miswired my SC's distributor. Running on 4 cylinders wasn't super fast, but it ran. These engines have enough power on five for sure.
Is it possible to tap (is that all that is required for a plug hole saver, or do you need to drill first?) this in the car? #6 is just about as inconvenient as #4. I am dubious.
I would think driving without a plug in a cylinder would cause problems with the mixture - you have a path through the intake runner to the air box, and the air which gets up into the box that way is going to lean the mixture quite a lot, like any other air leak in the intake on the cylinder side of the air measuring system over on the right side of the intake. But maybe not?
One option is paying for some professional assistance, or at least opinion.
Having a shop help with repairs, temporary or permanent, may be the best approach. Sewickley in Pittsburg is the closest I can think of offhand, and Duane Smith knows these cars:
dsmith@sewickleycars.com. Steinel's, a bit SE of Cleveland, is a good Porsche shop also. Someone may know of a better, closer, place. I just know these guys from race officiating.
Like you, I'd like something more specific than "just don't drive it any more." That may be exactly the right advice, but some explanation of what and how further harm is going to occur would be helpful.
All I can think of offhand is maybe a lean mixture, and you could do something about that by seeing if you could remove the intake runner for #6 and blocking its connection to the air box. Though perhaps removing the #6 intake rocker would be easier and maybe better - there is a whole lot of stuff over by #5 and 6 to get in your way getting to that runner.
As long as the engine compartment isn't very dusty, maybe you'd not suck much dusty air into that cylinder?
The engine is going to be out of balance, but you can keep the revs down to help with that. A chance of twisting the crank would not be on my list of worries.
You say you disconnected the injector. Good move, for sure, but where is the continuous (CIS injection) fuel flowing to? Did you find a way of plugging the line from the fuel distributor?
But you really should check out the U-Haul option, so you have some idea of the relative costs of things. I have trouble imagining repairing that particular spark plug hole without dropping the engine.
Checking the other plugs sounds like a good idea, considering. I can think of two potential reasons for this one being spit out: One is that it wasn't tightened on installation, but that would allow you to screw it back in (with maybe just the top few head threads broken off)and you say it just spins. The other is that someone along the line cross threaded things. When you have the engine out (at least eventually), pulling all the plugs to check for thread integrity might be in order.
Are you a PCA member? From time to time Porsche owners caught in a pickle get ahold of someone from the local PCA region to see what help can be had. You never know - maybe someone with a trailer might take your car at least to a local shop.