Kestas, commenting on your last points:
* A flush isn't done with all the components disconnected. Google an AC flush machine manufacturer/distributor and look at the instructions for hooking the machine to the car. The flush is done in 2 circuits, one through the evaporator, exp valve, and interconnecting hosing. The other circuit is through the condenser and interconnecting hosing/lines. Flushing in the reverse direction is good. The only components which are disconnected and out of either flush circuit are the compressor and the rec/drier. The rec/drier always gets replaced.
* There are other reasons than component replacement to add oil. A system which hasn't been opened up, component-by-component, and has been flushed prior to, for instance, an R12-to-134 retrofit, will need an oil recharge.
* We have 2 trade schools locally for auto and truck maintenance. I called and spoke with the AC instructor at one of the schools. His comments boil down to one idea. Put the oil into the discharge side somewhere after the compressor, whether you pour it in to an open, disconnected hose or whether you use an oil injector and let vacuum draw it in (vacuum pump assist, engine off) via the high-side valve to a closed system. He preferred adding to a closed system under vacuum (but that takes a vac pump).
* Think I've worked this thread to death. But it's all been helpful to me to arrive at procedure I can believe in. Thanks.