Is it normal for a 2.5 year old to...

[puppetno213]

Have a breakdown if she can't watch her favourite programme on TV?

Decide she likes something then she doesn't like it then she likes it again in the space of 10 seconds?

To whine constantly, repeating the same request over and over again with no ending until she gets what she wants. And if you ask her to wait a minute she falls apart (rolling on floor and crying/fake coughing)

To resent any attention given to her younger siblings and nudge/push them when adult's back is turned? (Despite the fact she is almost constantly the main focus of attention because we overcompensated when dtwins came along.)

To refuse to be potty trained. If we take her nappies off, she insists on sitting on you to do a poo. So you get a poo in your lap, or on your leg. She can do it where she wants to, but she just refuses. If you don't let her poo on you, then she has a breakdown (rolling on floor, crying, getting very hot and bothered.)

Apart from this she is securely attached. Happy for us to say goodbye and we'll be back later, and very happy to see us again.

My two and a half year old does everything except for the pooing. Although she has been potty trained for a few months, the last week she has started wetting herself several times a day.

Will she poo in a potty?

It all sounds fairly normal two year old behaviour to be honest, it does get better gradually.
The thing that strikes me about your post is that you seem very anxious about her.
They're not "breakdowns" this implies that some serious mental harm is going on, they're tantrums, just a two year old's way of expressing frustration at a world she doesn't quite understand.
She may be picking up on your anxiety and unwillingness to cause a "breakdown" which could lead to more controlling behaviour.
The only thing that concerns me about the behaviour is the pooing on your lap.
You say " if you don't let her she has a breakdown". This sort of implies that you do let her some of the time then?
If this is the case then please just don't, it isn't healthy and needs to be firmly told that we don't do it.
Could it be that she just needs to be in nappies for a while longer?