Part of the trouble you're having is probably caused by NVDA not coming into and going out of focus mode properly on youtube. Here's how I conduct youtube searches. It works well for me.
Goto youtube.com and press the letter e to goto the search edit box.
Press the NVDA key + spacebar to force NvDA into focus mode.
Type your search term and press enter. It sounds like nothing happened, but the search is, in fact, complete.
Press NvDA + spacebar again to leave focus mode.
Press the letter h until you get to the search results heading.
You can then use the up and down arrows to scroll through the results.