In an effort to keep this all straight in my head, I put together this quick diagram. Does it look right?
I think I might have 1/2 and 3/4 backwards... I can check when I get home.

http://forums.pelicanparts.com/uploa...1409064947.jpg

There's a good chance the reason you still can't get into 1st is because a couple of those synchro teeth are jammed up in your slider blocking the motion of the gear.

Once I lay the transmission on its side, I was able to wiggle the shafts out easily.

Here is 1/2 in all its glory.
First, here it is in second gear engaged. Everything slides in nicely. Looking in the gap towards first, I don't see any issues but the 4 missing teeth.
http://forums.pelicanparts.com/uploa...1409093408.jpg

However she still won't slide into first gear position. This is as far as she moves. The blockage seems to be even - it doesn't seem to move further in on one side or the other. I can't see what could be in the way. Ideas?
http://forums.pelicanparts.com/uploa...1409093508.jpg

After disassembling the 1/2 shaft, I found I still can't pop the operating sleeve over 1st, although it will slide over 2nd.

I compared 1st and 2nd carefully. The synchronizer ring on 2nd compresses and the thrust block and break band inside it move a little when I mesh into 2nd. The synchronizer ring on 1st won't compress at all, so the operating sleeve can't get over it. Its break band and thrust block are immobile.

I tried removing the circlip on 1st, but could only get it part way off. After messing with this a few times, I discovered I had somehow made the synchronizer ring compressible again, and now the operating sleeve can engage with it.

I presume something is broken inside, so I'm going to keep trying to get that circlip free.

There are circlip specific pliers you can grab at Sears that will get it right off for you.
Sounds like the broken teeth allowed the brake band and stop assembly able to stack up behind each other stopping the compression of the synchro.

Got the circlip off.
The synchronizer ring had been rotated too far in one direction, and had even jammed the inward-facing thrust block at an angle. This is why the ring couldn't compress, and the circlip was so tight.

My rookie analysis is that the driver attempted a downshift and nearly took it all the way into first gear. This put so much pressure on the synchro ring that it jammed, and probably took off a few dog teeth.

I'm going to examine 2nd's synchronizer as well, once I get some better circlip pliers. I am thinking I will need new:
1) 1st Gear
2) 1st Gear Synch Ring, Circlip, Brake Band, and Thrust Block
3) 2nd Gear Synch Ring ("While I'm in there")

I'm not sure if I want to pull 3/4 off their shafts, as they have been working just fine.
If I was doing this at the track and needed to drive home, I could probably reassemble and drive it for a while, but I suspect it is only a matter of time now until the 1st gear synchro does worse than jam.

Do you think I really need new brake band, anchor/thrust block, and stop block? I'll look at them carefully, but unlike synchro's I don't know how to identify damage or wear.

you don't need 1st gear, just the syncro teeth. you should replace both syncros and the shift sleeve together for best results. with all the trauma to the 1st band and dogs, perhaps new parts are in order there too.

What John Walker said.

For general educational reference, here is what I'm looking at so far part-wise.
The items in green are what JW recommends I replace seeing as I am replacing their matching part on 1st gear. I plan to inspect all the parts for 1st and 2nd carefully tomorrow night.

http://forums.pelicanparts.com/uploa...1409231785.jpg

Oh, and what is the best thing for cleaning these up? For now I've been bagging everything to keep it clean. Should I just wipe them with my shop towel, or should I use WD40 or something? I understand I don't want to de-oil them or they would rust.

no swepco if you want it to shift easily.

JW, what fluid would you recommend? I see another person recommended Kendall.

I would just leave them in the bags until you are ready to reassemble. I like to use CRC Chlorinated Brake Cleaner to degrease (red spray can). Capture the residue and dispose of at your town's haz waste day.

Some of the bearings that are pressed on the shafts are supposed to go on clean and dry, so you'll have to degrease those. See instructions in Pete Z.'s tutorial:
Porsche Wiki

Don't forget new lock washers for the bearing retainer plates, looks like you were about to lose one of those nuts.

Mark

kendall.

I presume this is our host's "Kendall 80W-90 GL-5 NS-MP Hypoid Gear Oil". It is cheaper too!

Can anyone recommend the best way to get the dog tooth gear off? I am reading about a factory tool and a puller, but don't see it on PP.

Pelican calls them syncro hub removal tools. It looks like you smack it.

The factory tools use a puller. See here:
How-To: Porsche 915 Transmission Repair Tutorial Part 4 - Porsche Wiki

New dog teeth are pressed into place.

On mine, I bought the new parts and a local independent shop that I've paid to do other work was kind enough to remove my old dog teeth and press in the new ones for free.

Mark

that's the old tool. the new one is two plates that bolt around the dog teeth and you press it off. maybe a local porsche shop has one.

Parts are ordered! Now its time to sit back and clean things for a few days.