I think it could also be beneficial to make a change that would apply to group bounties. This change would be that you no longer turn in bounties for a specific act but have to complete all bounties on each act before you can turn it in. Then you just get 1 loot box with all the act's mats and items instead of 5. This would prevent people from cherry picking bounties in acts and stop people from turning in stuff when it isn't on the bonus act.
Solo players could then keep focusing on act specific bounties. If they don't turn in the bounty when it is on the bonus act, then it really hurts no one but themselves.
Ditch the bonus bounty act thing. There is more then half the time one moron that turns in the wrong act on propose or by accident.
Then all the time you spent is wasted. Or award 5 bonus cache on completion of the last act to get people to stay the whole 25 bounties.
This is also a good way to stop botters from farming one act.
Although I empathise with your frustration I fail to see how this would be in any way related to making solo bounties viable.
Said that solo bounties are viable, I have completed 9782 bounties on my own. The yield however is unrewarding compared to what four people abusing a mechanic can achieve; even compared to what four selfish and incompetent people can achieve not working together but just being in the same game.