2) What’s the point of sending them for hunts, field explore & other once retainer is at 80?

Make money with ultra low effort but obvisouly not very much but still.
As I do : send my retainers on 1 hour ventures. If lucky, they drop interesting housing furnitures and some are expensive.
If not, they drop gears. Then I turn-in gears to Grand Company for seals. And with those seals, I bought Glamour Prism and sell them to MB.
Furthermore, I bought token venture.

If really lucky because it's rare, retainers can drop gears/dyes that sold to MB around 200-500K.

3) 2 in more than enough for the majority of players. And you have to pay a monthly sub if you want more than 2.

4) No. But put them at iLvL 500 will be ready for EW. I recon LvL 90 and iLvL 500 will be good for doing LvL 90 ventures. As now LvL 80 and iLvL 400.