here is my rant about what i know.
from what i understand, the fuses between the front and rear batt are to protect from shorts. for example, if there is a bare piece of metal on the ground and it cuts your run between the batts, the fuses will protect your batts from shorting out and exploding/damaging the batt,amp,equip, etc... thats the reason you have to have one before each batt. if the short happens in the middle of the wire, then both batts will be affected and both need to be protected.
as for the fuse before the amps, if the amps have fuses in them, then you dont "need" to add an extra fuse cause the amps already have that protection necessary. if the amp doesnt have any fuses, then it is recommended to protect your equip.
when you have both batts connected, your music should only be taking power from the rear batt. if you have your music on and you connect a dmm to the front batt and see that the voltage drops, then you know that it is taking from the front and the back batt isnt sufficient to keep up with the demands. thats the reason why people always say to put the biggest batt in the back (so it doesnt affect the front batt)
and as for grounding the amp to the back batt, its ok to do so. and if possible, do a run of 0/1 neg from front to back batt and connect the amp to the back batt. it will give the back batt and amp better connectivity. if you cant do the extra run, then just ground the rear batt cleanly and connect the amp to the batt. you dont want to be drilling alot of holes to the frame of your car..
if anything is wrong, please correct me for i am learning to. kthanksbye
edit: ^ lawls. i like that diagram