The methods mentioned above... a funnel to introduce some solvent and then blowing it out are the same my Guru ... Tray at Carlisle Auto Air in San Antonio said they use..
I think the main thing is to keep flushing until what you get out is completely clean....
I like the use of mineral spirits and then the regular commercial flush...
I would not switch to the commercial flush until your mineral spirit fills come out completely clean...
Given what has been said... if you can introduce it and leave it a while I think that is good also... don't be in a rush on any part of the procedure for flushing the evaporator....