yes for pool heating they are plumbed correct.
yes if you add more you can raise the temp.
yes it cost more to add more panels but if you heat the pool with gas it costs too

you should look into evacuated tubes

When you say you want to solar heat your pool are you meaning heating it through the winter? Or summmer or both? When we lived in Arizona we had a completely unheated pool that we heated quite warmly with a simple bubble pool cover. We had to leave it off in high summer because the pool got too warm.

I plumb them the opposite way. I think your's is backwards. I don't k now how big of a difference it would make to change it but My installs all have the cold water filling the collectors from the bottom up.

The collectors are under pressure so I honestly doubt it would make much of a difference.

What is the angle of your collectors? If you have too much heat in the summer and less in the winter, you may want to aim them lower toward the south where the winter sun is.