I cannot believe that any life vest would be attached with a double knot and that the cabin crew would be able to explain how to tie a double knot properly, securely and tight within the space of the 30 seconds that the safety briefing lasts.
you're wrong. at my airline we tie the lifejacket in a double bow on the left hand side. we state it MUST be done up on the left hand side and we have plenty of time to do it up in a double knot. also our life jackets have only one tab to pull and it doesnt matter how hard we tug at it! but, we have also as cabin crew never had to inflate a proper lifejacket. so we dont know how hard you would have to pull the tab to inflate it.