So here is the post i wanted to react ... well lets do this now:
"... so no using the random boats that litter the shore then?"
As far as i know, every boat that is on the shore is damaged and taking water ...
Not exactly best means of transport. :-/
I also dare to presume that when they went to fishing they didnt take hammers, nails and paint with them to fix their boat, or use the other ones as a source of raw material.

We don't know that. I also find it hard to believe that the nautiloid got EVERY boat. I also don't know any fisherman who doesn't bring a basic kit to survive for a day or two and to fix a boat. It's life and death.
The said fishermen also notice the tadpole and scream and run away
As far as i know (and i do, i just watched the scene few seconds ago

) they dont notice "the tadpole" ... they notice that they can "feel you" just the same way they felt that Mind Flayer just few seconds ago ... its logical to presume that Mind Flayers (and their tadpoles aswell) create some kind of psyonic field that resonates with others.
BTW
Duergars will recognize it exactly the same later on Grymforge.
Again, why would an NPC notice it? Mind flayers are rare, and they're not really aware of what just happened. Furthermore, Duergar are exposed to the Absolute's Chosen, and some of them ARE part of the Absolute's team. The fishermen aren't. I'd expect the druids to notice it more, but no.
The illithid decides not to slurp down the fishermen's brains but instead waits until the PC comes along (stop being a picky eater, illithid. You have 3HP left).
Illithid was too weak to force anyone get eaten ...
The fishermen were lured before you, and it's extremely likely their INT and their WIS are lower than yours (plus they don't have your companions shouting "What are you doing?"). No, the fishermen were far easier prey.
Astarion whines about not having seen his face in 200 years when (in my playthrough) he has a scroll of mirror image in his pack (and yes, it works on him).
Im not quite sure how this magic works ...
If caster (aka the person who gives them their look by projecting his own) dont remember his visage ... what would we see? And more interestingly, what would HE see?

The spell doesn't work that way. You can be blind and still cast mirror image.
Also, why would a rogue with an INT of 11 be able to read a book that is apparently difficult for my wizard? Etc.
Why wouldnt he? O_o
I mean last time i checked reading isnt skill that is reserved for most intelligent person in party ... nor skill that would require tremendous intelligence far beyond capabilities of meere mortals.

Because the spellbook is not written in Chondathan. And if you're a mage, the narrator tells you that people have gone mad for reading lesser tomes, indicating that you need certain level of arcane training to read it safely. The entire class system is a bit wonky in this game, because my mage should not be able to learn and cast bless. that's definitely a divine spell.
Try as you might, but looking at the lore of this game closely will bring up a lot of inconsistencies, which, for some people, break the immersion.