If it going backwards then it won't get damaged. The chances are, the damage has already been done!
Yes, their should be a shim, unless it's turbo plug. Try this:
Take the plug out of the engine. Have a good look at it. At the bottom of the plug, where the thread ends, does it bend inwards slightly? Place it on the table resting upright as it would do in an engine with the thread at the bottom. Does it bend in slightly at the bottom or does it cut off imediately? If it cuts off imediately, then it is a simple case of you having lost your washer. In which case, buy some assorted glow plugs ranging from hot-cold and experiement with which is best.
If it does bend in slightly, then you have a slightly more complex situation. The plug is called a turbo plug, the little rim around the edge gets over the problem of needing a washer. Here you have two options: either you have a faulty/wrong plug for your engine/fuel combination, or, you have the wrong
type of plug. ie. you should have a normal plug. Now, using the internet only that is as far as I can help you, everything else will be down to shop owners/fellow racers, but that should answer half the question. Again, if you have any problems, post them here and I'll try my best to answer them.
With regards to the ignition timing, this could be down to having the wrong type/temperature of glow plug. If the fuel ignites too early in the chamber, then the engine will not make a full cycle and what will happen is that the piston will go on it's way up the chamber to make the full cycle, then get blown back down, the same direction it came. This will produce a very rapid up-down-up-down, but not up-cylcle-down-clycle-up-cylce-down-clycle. ie. Imagine it like one of those pirate ship rides you get in theme parks which rock back and forth very high up, it is like that. A normal engine would rock all the way round, but this one only goes half way. This could mess up the clutch and get the car to move backwards slightly.
If the ignition timing is too late, then the engine will lose all of its power and it will struggle, possibly sound either choked or drowned, and may cut out if throttle is applied.
Hope that helps.
Thanks,
Chris