Hi! I do have some of the answers you seek! :)
I had a really hard time finding that exact fish myself! I eventually had to look it up. As it turns out, the river in the forest (the river in front of the lighthouse, which ends in a waterfall near Aunt May's house) is the one where you can find this fish, as it is apparently slower than the rest of the rivers on the island.
I don't think there is a passageway in the river area you mentioned. I was also confused by all the boards around there, but I don't think they have a function. I tried to enter through the top of the river, but it is not a passageway.
As for the icy passage, yes, there is another end of the tunnel! =) If you go to Aunt May's house, then north through the water (just staying near the edge of the ledges will do), it won't take many seconds until you reach the other passage! Hope you find it :D
And I've also been wondering what happens if you get all the bouncy flowers to bloom! I've been trying to find them all myself, and I'm not sure if I have found them all. I don't know if anything happens if you do get them all. I did a google search and there doesn't seem to be any guides for this yet, unfortunately.
I've seen other people having problems with accidentally interacting with things during the parkour races, and unfortunately I can't help with that as I never really had this problem. The technique I tend to use is to fly as high as I can from the get-go and swoop to build speed while I float over all the obstacles. Maybe that would help! (Tip: get the silver feather(s) first for better height and climbing speed to maximize this advantage!)
I hope all of this helps! :D Good luck!