The 3 previous seasons awarded a 150$ prize for 1st place but this season has $40. Why is this?

No money!

Many of us predicted this. People will not re-invest their own money into it and it will become a group of good players with all the coins.

I'd like to see statistics on the overall distribution of coins now vs beginning.

If more people invest into this season will the prizes rise or is it already set?

I'd guess that the prizes are already set.

Is the coin leaderboard all coin games (there are no conditions/limitations on them)? It would be nice to see total coins on there (ignoring season).

It would also be nice if total coins showed on a person's profile somewhere. It shows each season (but I assume you could hide that like ladders, stats, etc.