I can totally relate to what you're going through. My 5 month old does all that you've described, and I have to say it can be really confusing as baby can give 'mixed messages'.
Like you, I was led to believe that you shouldn't let a baby drink for less than 10 minutes or you will reduce the milk supply. Sometimes my baby goes 2 hourly between feeds and sometimes only 1. But it doesn't seem to make much difference whether he latches for 5 minutes or 35. He still looks for a feed again pretty soon after, and really doesn't seem to be made any worse with a very short feed.
Today he was squirming and biting again, and my Mum commented that she thought he might be a bit hungry still. This was after I had given up on the feed (sore nipples and discontented baby), and yet baby was crying and fussing loudly, which made me believe my Mum could be right.
I filled a bottle of formula and he wolfed down 7oz and looked for more. Afterwards he was happy as larry and a completely different baby. I felt so confused and upset because I treasure breastfeeding and loved doing it for a whole year with my first baby.
I don't like the idea of formula for a lot of reasons, I find the extra work of sterilising bottles and mixing feeds a real pain. I am trying to see is in the light that I am going to wean him now and so am trying to see the formula feed as a "wean" between breast feeds.
I am sorry for your stress about it all. I am feeling the same way and so want to thank you for your thread because there is a comfort in knowing that this is not perhaps so uncommon after all.