I came by here a few times once upon a time, and streets just aren't something I'm that excited about, so I had stopped frequenting. All these street mods are beautiful, for sure, but the only issues I've ever really had with road textures is the mess that sometimes occurs when you drop RTMT on NAM turn lanes. Then I found your link in Orange's thread, and WHOA! TREES!

I'm also excited, because if you're making a tree controller with these trees, then I don't have to. Not that I was progressing terribly far with it, but it was something I pick at every once in a while. Anyway, I'm happy to see how you've progressed with the tree controllers, and can't wait to see the final product.
I noticed a few posts back you figured out the ease of managing the data with Excel. .csv files come in particularly handy, because you can readily swap the data back and forth between spreadsheets and plain text to paste into Reader. As far as keeping track of all the changes and updates, a lot of that can be handled by equations to make it less tedious. Speaking of programmable spreadsheets for tree controllers,
have you seen this?. I never used it to really create anything, so I can't vouch for it's accuracy, but it uses Java and Excel together as a data management tool. It seems to mostly be aimed at tweaking existing controllers, but I think it could be useful as well at least for troubleshooting new ones. Providing all that Java works as advertised, which as I said, I haven't tested.
I'll cast a vote for a denser tree controller as well, something with coverage similar to c.p.'s Olympia controller would be awesome.
Onto some of your questions/issues:
#1. The small trees actually look good there. My best guess is that that particular location has a height/moisture condition that favors those trees - though if you've controlled for that already, I don't have any other ideas.
#3. Not so bad, I like the realistic effect of creating swampy areas.
#5. Meadows are tricky in that the models are so small they can get lost in the terrain texture. Adding the occasional small tree might counteract this without losing realism. Your later mosaics look like you've taken this approach, and they a good bit better than that first meadow shot did.
Those last few mosaics look beautiful, and make me wish autumn would hurry up and arrive.

Looks like you've got things in pretty good shape, I'll definitely have to keep an eye on things over here. Good luck!