Let's say here's the situation:

1) Car A (white) is going to make a U-turn.

2) Car B (black) is going to make a right turn.

3) Both cars do their actions together.

4) Accident happend.

So my questions are:

1) What fines and demerit points each of them receive?

2) What insurance companies will tell to each of them?

There's not enough information here. What traffic controls are there at the intersection? What the layout of the intersection? What colour are the traffic lights if there are any?

1) This road has 2 lines in total (one for each direction).

2) There are no traffic lights on the intersection (the right of way is determined by the STOP sign only).

3) There are no extra signs - left turn and U turn are allowed (not forbidden) on the intersection.

4) The accident reason - car B assumed that car A is going to make left turn instead of U-turn, so car B started the movement before car A was able to complete U-turn.

It might be easier if you uploaded a simple diagram and indicate where the stop signs are and the vehicle direction.

Hmmm... My initial post already contains an image, but for some unknown reasons only I can see it (I mean, when I'm logged in). That explains everything lol. Let me try again.

Let's say here's the situation:

1) Car A (white) is going to make a U-turn.

2) Car B (black) is going to make a right turn.

3) Both cars do their actions together.

4) The accident happened.

So my questions are:

1) What fines and demerit points each of them receive?

2) What insurance companies will tell to each of them?

I'm not sure what the law would say, but for insurance purposes the car making the u-turn would be 100% at fault (see section 19):

https://www.ontario.ca/laws/regulation/900668

As long as the driver of vehicle B complied with the requirements of the stop sign, I would totally agree with the above and may not even lay any charge.

How car B can be complied with the requirements if STOP sign dictates "wait until the way is clear before entering the intersection"? The right-of-way on this intersection belongs to vehicle A.

You may want to read section 136 of the HTA. The requirement is to stop. Then, "shall yield the right of way to traffic in the intersection or approaching the intersection on another highway so closely that to proceed would constitute an immediate hazard and, having so yielded the right of way, may proceed."

(2) Every driver or streetcar operator approaching, on another highway, an intersection referred to in sub section 1, shall yield the right of way to every driver or operator has complied with the requirements of sub section 1.

Driver A didn't do this.