> When does one decide enough is enough?

I stopped after 5 coats, seems OK so far.

> In between coats, it's recommended that I ...

Hopefully before you started varnishing you had a smooth surface. Then, after each coat just do a light sanding with the 400grit to evenly take off the shine and any little nits.

> Epifanes says to thin the varnish...

I used Captains varnish so it may be slightly different but - I found that warming the can of varnish for 10 minutes in a bowl of hot water (hot from tap, not boiling or anything) then opening it and pouring thru a strainer into a cup, then painting from the cup resulted in it flowing better and having fewer nits.

> Epifanes says you can spray their stuff...

Don't know

> Finally, what treatment for the very last coat?...

As someone else said, it's going to get scratched anyway, but I couldn't stop myself so I sanded the final coat with 600, 800, 1200, and 1500 grit, then polished it out with 3M Finesse It compound, then 3 coats of marine wax. It makes a beautiful finish. However, it gets scuffed up the first time you go out, so don't go to all the trouble if scuffing all your hard work is going to upset you.

> Is wax or any other 'final' coating recommended by anyone? Automotive poly
> coat stuff? WD-40?

See above, went to West Marine and bought a marine wax made for varnished boats.

> From time to time, someone will ask if any non-varnish...

No good answers here