1. Only from the above.
2. It's possible to get into the other side.
3. Not unfortunately :(
4. ^ See 2. ^
I'd suggest you to try it again,
no one has made it yet.
*spoilers*
Got in the tunnel! I tried calling the bird over to ride over the waterfall-pit but no luck crossing yet. I did potentially discover a bug. Upon returning to PUIJJO from the HUOM screen, the top bird was permanently gone (see imgur link). I'll see if it's reproducible.
There is a little "air shaft" in the secret tunnel that leads up....maybe I'll jump up, land on the bird, and jump again to the screen above PUIJJO. To be determined...
https://imgur.com/a/M8iEJWx
I take back my comment about the controls. After playing more and getting the hang of things, I think it's at a good level of difficulty. Since there's no enemies, there should be a challenge somewhere, and completing some of the harder jumps is definitely rewarding!
Ah, the missing bird bug resurfaces. I've no idea atm why and how it happens. :D What I've gathered is that the bird should return after the player goes a few screens away and then comes back, but it's still a guessing game...
Oh, the vertical air shaft? I thought THAT was the hole next to Mt. Sus you were asking about earlier :D it's not important (yet).
hint: there is a hidden path nearby to get past the waterfall pit.
Also, nice to hear that the controls "clicked". I'm still pondering if I should tweak them a bit, but this reassures that platforming physics are at least 95% complete.