Geared up my toon a bit now so want to sell these A1-5 T8 bounties, want to know what's the trick to get all the bonus caches, and what's the bug people are talking about.

10fg for anyone who gives a proper guide

Do all bounties,leave 1 quest from act 3& act 4(this is bugged since u get them both from bastion keep tyrael,u wont get bonus cache).Invite ppl in game get bags order to get bonus bags, complete remaining quest order from act 3&4.profit

Does that mean we have to fish for the Bonus act to be in 1-2-5 (or any alternative) order?

the order doesn;t matterwhile you are clearing seeing as you will be doing all of them. you simply hand in to tyreal while in the right act and then it selects the new act. If all bounties are complete it's automatic 5 bonus for turning in the right order, which you will see on your map.