Hi there! Some responses and suggestions:

1) They are useful for anyone

2)Passive Money maker.:
convert junk dungeons drops into Grand Conpany currency, use those to buy ventures and then send your retainers on 18 hour ventures. You will quickly accumulate lots of allagan coins which is cash money. Even more if you funnel these coins to the Doma Restoration.

3) No; they are useful for storage space. You don’t need more than 2.

4) ehh. As long as you can send them on the highest possible 18 hour thing it doesn’t matter. Or you can gamble with the other ventures.

Bonus)

I recommend you make your retainers the same job if your plan is to maximize income, and furthermore suggest the job match your main (so both are paladins or fishermen)

If you are paying several months in advance (I forget the plan name, but not monthly payment) you get up to 8 characters on your server. You can easily level fisher to 80 in like a week and as long as your alt can run a few dungeons solo you can easily gather items for ventures for more profits.