Changing antifreeze in 1996 Corolla

[GSS123]

There doesn't appear to any ideal location to plumb in a coolant system flush kit. All the factory hoses are very short with little room to splice in the flush adaptor. So I was planning on removing the engine coolent by removing the block drain plug near the oil filter. The plug is not moving at all. I suspect that it has never been removed and the alluminum block and steel plug have become like one unit. Anyone out there fought this problem and won?

I may just drain the radiator and reservoir and call it good? But would like to drain as much of the old coolent as possible.

Second question should I use dealer supplied (Toyota recommended) antifreeze or will any good aftermarket type work as well?

131,000 miles and going strong