Neither of us has had much luck of late.
Identifying your problem I think it is one of two main candidates and they are related. First of all dried yeast does not need aerating, as has already been pointed out. Whisking air into the wort does indeed whisk in whatever is in the air. Now, if you have enough yeast pitched it can overwhelm whatever else is in there by sheer numbers. Problem is Chris White, of White Labs fame, has suggested pitching without hydration can kill up to 50% (see his Brewers Publications Yeast book for reference). I grant you Lallemand have recently suggested it doesn't matter but they're in the business of selling simplicity to encourage buying. Oh, by the way,
yeast doesn't actually require oxygen to ferment. , it needs it for other reasons, particularly if you are looking to harvest and repitch wet yeasts and I wouldn't brew without doing it for those reasons, but I use pure O2 in a closed system to avoid the atmosphere adding stuff I don't want. I'm not convinced liquouring back was your problem either, however I would use RO water or its equivalent as your tap water may contain high levels of alkalinity raising the pH of the wort, not so good for the now potentially compromised yeast either.