Scrum should only break when the ref shouts that it's out the back.

I have to admit the "fumble" interpretation (i.e. the player lost it but it clearly went backwards) of the knock on drives me mad, it's just stupid, might as well just call all fumbles as a knock on rather than the current situation where it's pot luck whether the referee allows play on from a fumble backwards or pulls it up.

Back to six after a charged down kick.
The emphasis should be on the side in possession to get their kick away. If they fail to do so, that's either bad play by them or good play by the opposition. If you regather the rebound you certainly have not 'earned' another set so the tackle count should continue (which could lead to some exciting off-the-cuffery).
Another advantage to changing this rule it would mean the ref would no longer need to make the sometimes difficult distinction between a chargedown and being accidentally struck with the ball.
It might also discourage the almost inevitable grubber when you're near the opposition line on last tackle if an opponent could stick a foot out to block it without potentially giving another set away.

Not really, if a player is carrying the ball and he passes it backwards and it goes to ground it is not a fumble. If a player who is catching the ball fails to take it cleanly and drops it then it is a fumble. The only problem would be if a referee can't tell the difference between a player catching and a player passing. The only time this would be an issue would be if a player bats the ball rather than catches and passes it, in which case it's a fumble if it hits the ground, and not a fumble if another player catches it.

At the moment it's hit and miss whether a ball knocked backwards is going to get play on, and that's very frustrating because it's inconsistent and because it's a poor execution of the laws.

I disagree, a charge down is not just making a play for the ball, it is actually playing the ball, so whoever comes up with posession is entitled to another set. There is no issue with the interpretation of a charge down itself, an actual charge down is a pretty clear and deliberate act, the issue is the borderline between accidental contact and deliberate contact with a kick, but there are probably a few ways of clarifying and making that more consistent.