As others have said, it is likely the barrister will be allowed.
I didn't know until the morning of my hearing if my barrister or my solicitor would be able to represent me (because anther case my barrister was acting on had been adjourned to the day of my tribunal). We had change requests going back for a week before because of this. On the day, my barrister just turned up (because his other case had been adjourned again the afternoon before and there hadn't been time for another change request to name him). The panel let him in without even mentioning it.
Also, it can be good too for the LA to have a barrister. I know someone who went to a 2 day hearing - day 1 with the sen officer and day 2 with a barrister. Day 1 was a nightmare because of the LA's inability to understand the law - let alone being able to apply it! Day 2 (with barrister) was a lot smoother because even though the barrister was obviously acting for the LA, he was working within the law and understood the law. The LA weren't able to do anything stupid or illegal with their own barrister reigning them in. It also stopped the hearing going into a 3rd day - which would have happened without the barrister overseeing the LA's behaviour.
So the LA having a barrister could actually be a good thing
The SEN officer is another matter. You should refuse to allow her as an observer. But if she isn't allowed in the room as an observer, there would be nothing to stop her being in the waiting room. So the barrister can take his orders from her. My Sen officer tried (48 hours before hearing) to be 4th witness. we put in a heavy objection but my barrister requested that she was in the waiting room because she wrote the statement. He thought it would be good to have her on the premises to stop any attempt at an adjournment if something in the statement wasn't understood.
She was denied being a 4th witness by a judge the night before. But on the morning of the hearing, the LA put in another request for her to be the 4th witness and this time "in the interest of justice" she was allowed in as the 4th witness (to my 3!) yes that's what the judge said "in the interest of justice"!!.
So you could fight now for the sen officer not to be an observer etc but only for it to be overturned on the day.
Personally, I would object to her being an observer as it would be pointless her being present in that capacity. But as she wrote the statement, I would put in a change request that she is YOUR 4th witness on the grounds that she wrote the NIL so should be present in the room and therefore able to account for her actions during the trial hearing.
Ici says that you're supposed to give 2 weeks notice. But my LA tried to do it less then 48 hours before the hearing and then again on the day of hearing. You could put in your change request the reason being that it's so late naming her as your 4th witness is because you always thought she'd be present. And she intends to attend the hearing anyway so she's obviously available that day.
This is now tactics from your LA - they know they have a hopeless case so they're just trying it on now