Hey Rolf,
1. First time I hear of this. Best thing is to phone Didima Camp and ask them. I'll try and find out more on this too. I'm guessing the smugglers have forged a new route through the rock-fall by now!
2. Have never heard of any issues, and I don't have any concerns leaving my car there. Bear in mind that if you are dropping a car off there and then driving across to Didima, you will pay the fees at some point and will need the receipt to prove at either end that you have already paid.
3. Accommodation size is seasonal, because as plouw mentioned, the cave is notorious for getting wet (other caves get wet too - just that this one is popular and thus has more of a reputation). I've used it on a number of occasions, but never in the wet season, and so my experience of it has always been good. When it’s dry it is excellent shelter (have felt snug in it through icy temperatures, howling winds and snowfall).
In good conditions it could accommodate 6 in the main sleeping area and even more if you are prepared to sleep in the areas which are less favourable. I find these ratings to be pretty subjective because preferences vary and people have different ideas about what is acceptable. In very wet conditions it could border on being unusable.
Years back the main area was in the furthest corner, and the area directly behind the rock wall was very wet - now this seems to have reversed somewhat, and the cave is a little more spacious.
The use of a ground sheet is recommended in any season - if it's not damp, it's dusty!
It does get used by the smugglers sadly and the effect is noticeable. I was up there last in September last year and we did a major clean up starting in the cave and all the way down Gray's Pass. We had bags full of rubbish strapped to our packs! plouw, you are quite right - Gray's has developed into a pretty major smuggling and cattle rustling route.
You mentioned that you are going "in the next few weeks". If you go soon the timing will be good - things will be fairly dry still, the smuggling will slump a little and there will be less shepherds around.