I would like to think that these gearboxes are built in such a way that two gears could not be engaged at once, but if it's at all possible it would be reverse and another gear.

I read your post carefully and it sounds like when you went for first, you engaged reverse. Then when the car died, as you pulled it out of what you thought was first, you disengaged, or popped something loose in the shift linkage. Then when you tried to go for a gear, I am assuming the linkage was not working and the car remained in reverse. I have had a similar experience. Once under the car, when you manually shift the gearbox into neutral in all gears, the linkage gets re-aligned and may be usable. However, the wear on those linkage parts will cause the same problem again. The most likely place for this problem is where the bottom of the shift lever slides through gates in the linkage rods. Worn, they sometimes slide right out of the gate, making the linkage useless.

Of course, all this is WAG'ing, but what you describe sounds so similar to the problem I experienced.

If the trans has had recent work, suspect that also. It's also likely that when the linkage was off for the rebuild, it may have not been adjusted well when replaced.

I doubt you did anything serious to your trans,or you would have heard some horrific noises, so look that linkage over carefully. I'd be interested in knowing the outcome of all this. Please post what you find. See you on the board.