hun... TBH hun this sounds like a normal stage to me, my A is going through similar at the moment.
They are at an age now where they are trying to push the boundaries, they have a voice (IYKWIM) and are learning how to use it.

At the moment i am going through the same thing with tea, she eats a bit then asks for a drink, no drink she cracks it and wants to get down.. even if she was enjoying her dinner she says NO and wants off.
Instead of letting her down now we strap her in (we have the travel seat on a normal chair) and wotn let her down until we are finished.. she's not allowed any other foods either.

The tantrums are another thing too, she's getting good at throwing herself on the floor, kicking and screaming.
I try my best to ignore her, i am giving her a warning like you stop that or you'll go in your room, she gets 2 warnings and if she doesn't stop i put her in her room.
The screaming when i do that is HEARTBREAKING but i have to follow through with the threat so i close the door and walk away and come back in a minute or 2, make her say sorry, give her a cuddle then go back to what we were doing.. it's a hit and a miss most days though.

If it's something your really worried about maybe speak to the MCHN, i have a horrible feeling they will say it's terrible two's... am going to discuss the same issues with mine when we go in a couple of weeks.
In the meantime hun your doing a wonderful job... when she's doing it take deep breaths and count to 10 BEFORE you say or do anything.. then you go at it with a calm head.

Sorry prob no help at all