Sir... Thank you, both of you, but really it isn't necessary. You have already done more than enough to help me. I value knowledge and experience. Sharing that experience ensures that others don't have to go through the same mistakes. You can't find everything in books - that's why I really appreciate your help and advice. Yeah, truth is, that right now, I'm a poor student who can't buy his own sailboat or even tools to make it, but soon I'll find a full time job, and then these problems will be solved. I guess this kind of experience - doing something with almost nothing - teaches to value things that later come without effort. Also, through mistakes and hard work you can understand these principles of the sailboat, see what works and what doesn't. I think it's way better than to get everything "one the plate" from the start.
And I'm thankful for that you value my determination and efforts. It's really something to have people that believe in you - for example, my family, which watches my attempts to make this sailboat, just laugh and say that I'll capsize on the shore.
I remember a joke we created when last year I was making my catamaran:
It is generally assumed that rats abandon sinking ship - it is good indication if the ship will actually sink. So, when making ships, I invented a "rat test" - put a rat on the boat that was just built, and see if it tries to run (just an idea, never actually tried it - didn't see any rats in 100m radius from my boat). My parents said that if I tried that "rat test" on my catamaran, best case scenario - rat would die fighting rather to step in it; worst case scenario - chew through the hull and sink it

Also, it is in best maritime traditions that captain leaves his sinking ship last, or goes down with it. Then I thought - what if captain has a pet rat? Would that rat leave ship last also, only when other rats have escaped?
Okay, so here's the news: I rigged the ship this morning (just took half an hour). Mast step holds firm and strong. I wouldn't risk raising sail without stays, but it holds firm enough for me to tension them.
Sail looks and performs beautiful. I'm lucky that without the boom, sheet goes along the CoE-clew line, result is perfect sail shape, with no twisting, wrinkles, etc. Even with these 3.12m^2, it has so much power, that in few gusts, boat launched forward with such force, that the water started to rush loudly at the bow, and I almost fell of my seat.
Problem is... There was no wind today. I should have expected it - when boat is not ready, there is a fine, steady breeze. When boat is rigged to sail - water is like a mirror. I spent 2 hours rowing through the lake, searching, but... "As idle as a painted ship upon a painted ocean", Samuel Taylor would say.
Also, there are still some problems with the boat. While mast and sail is perfect, in these few gusts I saw that boat is drifting heavily, making speed, but not to windward. I'm starting to fear that leeboards are too small. I'll attempt to improve them, to stand more firm beside the hull, since now they still move around.
And that damn rudder. It takes maybe around 3-4 minutes to lash the oar to the hull, and still it doesn't hold firm. If there were some heavy winds, in 3 minutes boat can drift a lot of distance. I'll start making some improvised rudder socket - maybe it will lash easier, allowing to set in the rudder in seconds.
So, still some work to do - but a lot of things are solved and work better than expected.
I'm attaching 5 photos, still ashore. Leeboards aren't in their places yet, because it's too shallow. In the second photo you may see that I decided to move stays lower - it'll improve their angle, so I don't have to tension them too hard, and since sail isn't raised, but tied around the mast, it doesn't interfere with anything. Also, I found I actually have some hardware, leftover from the catamaran, so I used it for rigging and mast step.
In the 4th photo, please admire that mast step (took some 5 hours to make it) - just a few wooden blocks (made from logs), and some metal plates. It is very strong, and it's lashed to the seat, while the lower part rests on the floor.
In the last photo you may see leeboards more closely, and the rudder (as I said, I found out it doesn't work as expected yet).
I'm continue to work on improvements.
P.S. oh, and don't mind that little board at the stern with black thing attached to it - that is sonar radar my father uses for fishing, and that board is way too weak to attach rudder to it.