Hey mate, been a while since I did the RB25DE+t conversion, but I'll have a go at answering your questions...
I took the water from the heater line on the firewall, just behind the head and slightly to the drivers side, just before the heater tap (which is just there, you'll see it, got a little cable moving the tap itself). Yes, it is this one which is the feed, you'll obviously need to add your t-piece in before the heater tap (i.e. on the drivers side of the tap) so tat it still gets water when the heater tap closes.
I returned the water via the standard turbo banjo fitting on the block - now I believe the S1 engines had the water return fittings on the block for both DE and DET, whereas the S2 only had it on the turbo variant, luckily I had a S1 so it was just a case of removing the stock blanking bolt and chucking in the stock turbo water return line one.
Hope that helps!