Take a breath. You can do this.
Firstly, you need to know exactly what has happened. You say the LA has finalised the secondary phase transfer EHCP. Then you say the primary is named in I, but don’t mention that the secondary is also named from September. You need to know whether you have the finalised phase transfer EHCP or not.
If you do have the finalised EHCP for secondary, you could request leave to appeal out of time. Not guaranteed to be accepted by SENDIST, but sometimes is. Appeal BFI, not just I.
Alongside this, go back to the LA to request the date on the letter is corrected. I wouldn’t assume the letter was there and you just didn’t see it before now. It isn’t unheard of for LAs to put letters on with dates months previously. Not unheard of, but it is less common for LAs to finalise secondary phase transfer EHCPs in November. The fact you got the message this week suggests the LA did not finalise the phase transfer EHCP in November.
You should have the finalised EHCP by now, but if you don’t, chase the LA. If that doesn’t work, you will need a pre-action letter so I would start looking for someone with capacity to take your case.
Look at your evidence. Consider whether you need independent assessments. You mention EP but what about OT and SALT? If you need assessments but can’t afford them and aren’t eligible for legal aid, there are charities who can help.
Representation for an appeal isn’t essential. Most parents aren’t represented. If it is a choice between representation and independent assessments, you should prioritise independent assessments. The best representation in the country can only work with the evidnece available.
DS hasn’t missed the chance of SS. It is still possible for one to be named.
Be careful with SENDIASS. Some are excellent but too many repeat the LA’s unlawful policies.
You mention not knowing your obligations. The naming of a school in I doesn’t mean you are obliged to send DS. It doesn’t automatically register DS at the school. I wouldn’t personally, but if you decide not to appeal you are still free to EHE or make your own arrangements at an independent school for example.