1) Use a +speed soup and beast form to jump up. Takes a few tries, but eventually you'll make the jump. There are enough fish around to restock on hot soup, which works for this purpose.
2) There are a few that i know from browsing this forum (haven't done any myself), so first, you can do the kelp forest before mithalas, but it's really a lot harder imho (did that on my first playthrough), and the real sequence break things, you can reach the veil without getting beast form (there is a current that requires beast form, to pass it: have nature form, find a rock, shoot a vine onto it, in such an angle that clinging to the vine will pull the stone - with you - through the current. There is a video on youtube about this.
About skipping forms: fish and spirit are not needed, but sun is. You need it for the last boss' eye, otherwise, its totally optional). Beast is definitely required, and you need nature at least to guide the girl in the sunken city. Probably more. And obviously energy for charging up all the pearls, that unlock doors.
3) Uff, just out of my head (there are more):
- shooting 4 or 6 projectiles (instead of 3) with a energy form charge
- Krill can, and will, if given enough time, overwhelm your PC - massive growth. The game does not like using dual form scream on a screenful of krill.
- You can roll to pull ingredients
- Fall damage does not exist, and you can not double jump (awww...

)
- Leeches can slow you down until you can't move anymore. At all.
- Some of the collectibles are SO devilishly placed that i found them the first time when i openend up the game in the editor, about 10 months after I got it. Never seen those all this time.
- Rainbow mushrooms are NOT normal mushrooms (but can be used like them, without any bad effects)
- Rainbow soup is awesome, and makes boss battles a lot cooler (read: COLORFUL and AWESOME and CHALLENGING). Kids, don't try this at home.
- Naija is looking at interesting things (even if they are about to bump right into her face lol), and smiling if it's friendly.
- There is a bug that lets you cling to big mithalas jellyfish even in fish form (just go ahead and do it. Super-speed jump-off!)
- there are only 6 or 8 red bulbs in the whole game.
- you take 1.6 times the normal tamage in fish form, 0.75 times normal damage in the crab costume, and 0.6 times the damage if you are wearing the seahorse armor AND riding a seahorse.
- eating pistol shrimps is hard, but awesome. If you manage to eat one and then use the shot OUT OF WATER (jump high in the veil), you will fall down very slowly (i think noone else has discovered this so far). Could try to use this at the high, rightmost spot of the veil. And it can be used to knock you back up. There you have not only double-jump, but multi-jump.

- you can shoot an energy shot every 0.2 seconds. Clicking faster has no use, so don't stress your fingers more than necessary.
- if you are in spirit form, you can't leave the water. If you try, you instantly return from the spirit world, at the place where you left, but still get the high velocity+direction you would get if you were leaving the water.
- you can absorb shots in spirit form. Go near, and right-click. Gives one point of health per 5 shots, but you will still be hit + damaged by the shots even in spirit form, so be careful.
- Li's shots get stronger for a time after dropping an ice cream for him.
- If your piranha pet eats something and you are in beast form, it counts as if Naija had eaten it. Means you get all shots and bonuses and all that. Don't know if it counts also when you are not in beast form.
- ....
- probably many more but that goes too much into programming detail and how the game was made.
4) No, some are never used, and can only be found by cooking the right thing. But the recipes are all in a single file if you want to spoil yourself...
5) Upside down jellys? Not really, but you can eat them in beast form. They exist for real, too. The big "walkers" are just there because they are cool. Like the big momma turtle, you can'tt do anything with them (earlier there was a bug that you could eat them in beast form, that must've been funny...)
Last)
Short answer: No.
[Edit Dec. 2013]
We're making an unofficial one.Long answer: Maaaaaaaaaaybe. Heard an interview with Alec where he said that the audience is there for sure, but there are other things to do first. So, don't count on it. No word on details about what he planned, just that plans werealready made and cool ideas and all that. Mysterious.
Personal answer: Why not. Modding always works, and afaik that's the original interpretation of "To be continued" - people making mods and extending the story, or making their own. Not much has happened in this section; however, be sure to check the forum in a few weeks, i might have something to show then

More questions? Need more detail/weblinks/proofs? Go ahead
I'm just really tired right now and can't think, so i'm stopping here for now.