Ditto on the quality terminal tackle ; I only use SAMPO swivels and snaps [ crosslock only ] : Small price to pay for not being " broke-off "
Ps. Dan Keating has great books on big water fishing ; starting with " Keating on Kings " .
PPs. Always check hooks for sharpness !
#1 150 ft. of cable is enough IMO ; 90 ft. down is usually my max down
#2 8lb. balls will work but will have " blowback " ; how heavy you want to go with manual riggers is up to you.
#3 Which release to use ? will get varied opinions ; I only use OFFSHORE releases since they came out ~ 25 years ago : They clip to the ball with a short cable . Those who "reef" down the rod line complain the line slides off the clamp . I do not like the SCOTTY or the BLACK .
#4 Not sure why you can only run 4 rods on a ? 19 ft. boat. Dipsy divers do get fish but require different setup with wire or braid line. The more line I run the greater chance of a tangled mess which can ruin a good day.
If you want more detail send PM.