Flushing Radiator Q's & Situation

[hellllo]

I was flushing my radiator today and fell into a situation. First the koyo radiator plastic petlock got stripped (why couldn't they make it a screw metal plug ) Since i couldn't get the coolant out from that way I went with a different procedure. I opened the lower radiator hose and got all the coolant out from the radiator that way.

But then i fell into another problem... I couldn't find the drain plug on the block. FSM says under the header but there's nothing there to unscrew! So I just tried to flush everything through the radiator while washing my car between each flush shift.

I flushed it about 6 times from the radiator with water and prestone flush solution. The water seems to be clear. But now I have no idea how to put full 100% coolant in the cooling system since I can only flush from the radiator. Currently its full tap water inside the engine & radiator, but i want all coolant in there (Prestone 50/50). How would go about this situation?

I dids:
-Used lower radiator hose to drain radiator
-Flushed using hose tap water
-Heater Full Blast after each fill until operating temperature
-Redo all these after engine is cool

I did nots:
-Drain engine coolant from block plug
-Use radiator petlock to drain radiator

Q's
-FSM talks about the air relief hole, What does it do? I've opened it and filled radiator up and water just spills out of there...
-Can someone help me with a picture for drain plug on the SR20DET?
-How long does it take for water to circulate through the whole engine?

I'm going to using Linsle Funnel System to get the air out of the cooling system when I do the final flush and refill with Prestone 50/50. I know tap water will cause rust thats why I want to do this the right way. Thanks alot in advance! <3