I"m not sure you are clear on what you are asking. What do you mean buffs from hogyoku pills? They act as a surrogate to swallowing fragments. It's a permanent increase. So if you save up a few you can use them all at the same time. Yalana answered your questions correctly. Maybe you were confused on how those work.
I understand that BUT, along with that they add a temporary boost. For example, I just used 2 hogyoku pills on the flawless physical attack jade that I have on my main. (Note: The jade is level 14 and did NOT level from using the pills) After that, my main's physical attack rose from 136k to 144k. That boost is temporary because my main's attack has since dropped to 136k again.
yes the bonuses stack and i think the duration for me once was 1,5hours when i faced aizen first minion thats how long i had the bonus stats and 3rd question no idea, probably depends on your lvl on that certain stat?