Thermostat stuck open - what are the implications?

[nitroman]

Hi all, only started doing maintenance on my bug recently and now looking into a coolant problem. It takes a long time for the engine to warm up (temp light is blue), at least 10 minutes. Reading these forums and understanding more now, I checked the upper radiator hose and sure enough it's warm, so that means that coolant is flowing into radiator which it shouldn't yet (is my diagnosis correct?).

So the water pump, coolant and coolant temp sensor was replaced 4 years ago at 75,000 km, but the thermostat wasn't. So I'm going to try to replace it but first some nooby questions:

1) What are the implications if I don't fix it right away? I'd like to wait till it's a bit warmer as I only have time to work late at night. Though, it's been like this for a long long time so I hope I haven't done any major permanent damage to anything. I always drive it slowly when it's not warmed up, never get past 3000 rpm.
2) Do I have to drain the coolant first? Is there anyway I can replace it without draining? And if so, is draining it from the nozzle at the bottom of radiator good enough? I mean I don't have to drain from the oil cooler hose as well right? I'm not looking to do a full drain since the coolant is still OK, so I'm fine if some old coolant is left in there.
3) Is it safe to reuse the old coolant or better to put in new?
4) If I'm draining only from the radiator nozzle and not a full drain, how much coolant do I need to buy?
5) Which pentosin coolant is it exactly? There's Pentofrost SF and Pentofrost++. The Pentofrost++ is much cheaper, can I use that and mix with my existing coolant (it's red, so I think it's G12 since it was changed at the stealership). I think the ++ has silicates, so does that mean it's a No-No? I know I can also buy from VW stealership but due to some bad experiences I'm not giving them anymore of my money .

Thanks.