If you miss a critical question...well, here's what I do. I go out to my car, go over my notes/questionnaire to make sure I hit everything I was supposed to. If I didn't, I either call the store and say "oh, I was just in, and I just have one quick stupid question, would you tell me please......?" OR just walk back into the location and ask the question. If I really, really REALLY screwed up, I reschedule the shop.
As for "leading"...just SHUT UP. It takes practice, I know. Shut up, look at the guy, raise an eyebrow like "hey, buddy, what's next here?"; most people can't BEAR the silence, and will soon start talking. THEN you lead.
You actually don't ask your question until you're absolutely certain the salesperson is done but has not covered the salient points. At that point, I say something along "but I read in Consumer's Report that....."...or "do you know how I can check to see if there have been any recalls on this?".
Videos on youtube are mostly aimed at people picking up jobs at base pay. I really believe you can't make a living doing this on $4 shops.
The Penny Hoarder has several excellent articles on shopping.
You can also take Jacob's free e-zine about shopping.
And HAUNT, HAUNT, HAUNT all the sticky threads in the new shoppers section.
Oh, and stop "estimating" which question is important to the client. Treat every single question as of equal importance -- i.e., if you don't get an answer to it, your report will be unacceptable. That means if you're supposed to have safety features explained, and the guy doesn't touch on them, you MUST ask, "what safety equipment do you sell to go with this?" and if they don't sell it at all, you report exactly that. "They don't stock or sell any safety equipment, so the salesman was unable to go into safety equipment in depth. He did say the safety equipment is available at Walmart, and doesn't cost much."
That's just me, though. I hope other shoppers will contribute their 2CW!!!!!