If you don't have a transponder then that's ok.
No. If you don't have a mode-S transponder you are confined to the airspace below 1200' (which is a bit awkward for gliders) or you are limited to a few Transponder Free Zones - little circles around well-known glider sites. You can forget about doing any significant x-country work unless you manage to pull that off below 1200'.
If you DO have a transponder then you're required to turn it off when below 1200' or when being launched.
Again, no. If you do have a transponder the principle for all aircraft (powered or otherwise) is to have it turned on as soon as you enter the runway ready for departure, and to turn it off upon leaving the runway. The exception to this is a winch-launched glider. They have such a high rate of ascent that TCAS-equipped aircraft several 1000s of feet above will do an extrapolation and give a warning or even a mandatory traffic avoidance resolution. After all, TCAS does not know that the winch will run out of steam at 1500' or so. And that TA is not fun for the SLF in the back. So the advice given in the AIC to gliders specifically is to only activate the transponder once the launch is finished. Obviously once it's on, you leave it on until you're well and truly back on the ground in the landing field. So yes, it's on while thermaling and it's on in the circuit, even below 1200'.
With the introduction of Mode-S, secondary returns (i.e. transponder squark codes), are filtered by ATC to instead just show primary returns on their radar. Don't know much about radar but I didn't think that primary returns show anything other than location, i.e. don't show height and so ATC can't say if something is above//below you. But you're happy simply to know that there are gliders in the area...something that most radars can already achieve without even the need for gliders to be Mode-S equipped.
Again, no. It's the Air Traffic Controller him/herself who can control the filtering, not some anonymous programmer in a consultancy firm far away. So if there's a gaggle of gliders it's the controller who elects to see less information about them on the screen. Generally known as de-clutter. But as soon as detailed information is required, it's there. Courtesy of mode-S.
Perhaps the Dutch implementation is a good opportunity for the various other authorities to learn how NOT to implement such requirements.
I agree that other ATC providers can definitely learn lessons from the way the Dutch handled the mode-S implementation. But your post is a bit too simplistic for that.