So every now and then I'm having this problem where my pose doesn't look like the same in Blender and in game.

For example the current situation:
I tried to make a couple pose where the other sim is sitting on a chair, so as you know I had to lower down the root joint to get him on chair. Everything looks as great as can be in Blender, BUT: In game my sim doesn't sit on a chair, he ends up hovering in the air, as if he actually took the pose correctly but the root joint didn't lower him down for some reason.

I tried to fix this by moving the root joint down just a slight bit in blender and importing my file again, but it didn't work. I've had this problem earlier too and I fixed it by just making a whole new pose. But you know, it's darn frustrating when you got a great pose, it doesn't work and you just have to do it again... and you never know when it's gonna work and when it won't! So is there any way to fix this problem? I'm really giving up with pose making if I have to continuously be afraid of that all my work is in vain and I have to make my poses again and again and again...

When it comes to in-game poses, you should be sure that your sim isn't using a 'height slider'. I had this issue a lot with sims who used height sliders and I ended up using the 'move' option that comes with the Pose player addon.

Else you can take a look at this post: http://modthesims.info/showthread.php?t=580827

Not using any height sliders.
If I use the move option, when I move sims down to 3 they turn dark. So that's not a solution as my sim floats way too high for that.

It seems, in that case the solution was downgrading Blender from 2.77 to 2.67. I have 2.67, never used anything else. So that can't be the solution either.

Problem solved. Simply appended another rig and via copy-paste copied the joints on the new rig. After copying the new rig was floating in air so the problem was on Blender itself too. Lowering the root joint on that new rig worked.

I'm glad that worked out for you! I started to have issues with 2.69 when working on facial expressions on poses. Or the pose itself! They got pretty stiff-looking at some point.