you should be able to reach the 2 heater matrix pipes from the bulk head inside the engine bay (behind the engine) on my touran they are not the easiest to get to but more than doable.
you may have to take a couple of minor bits off to gain better access. as for the video I see.. I believe it was this in link below. pretty much explains everything. but I would flush the pipes both ways starting from the outlet hose. the water from the hose/garden tap will force/push the water through.. its in the video, you'll see.
the coolant leak under the bottle could be from the backup of coolant / pressure inside the system pushing coolant out the overflow. (unless I'm mistaken, I'm sure there is an overflow)
as for the coolant return pipe.. on the coolant bottle, youll see a small black hose going to it, this is the return pipe. disconnect it and see if waters coming out.. or if your bottle is clear enough and not manky, just see if you can see water entering the bottle from that pipe.
also, once you've done the clean up etc and are happy.. top the coolant/anti freeze up to the correct level and start your engine leaving the coolant cap off.. run the heating on hot with fans going. give all of your fatter coolant hoses around the engine a little squeeze too, it might help blow some air bubbles around and out! once the engine has reached operating temperature or there abouts.. you can put the cap back on. they are self bleeding systems, but I think the lid has to be off.. I've always done it this way with no issues.
defo sounds like you have a blockage somewhere to me.
hope this helps.
