If this is a sixth form at a state school, they shouldn’t be off-rolling DD just because her health means she can’t attend full time.
Some people do choose to request another EHCNA rather than challenging the Tribunal’s decision. You can request another EHCNA now if you wish.
I think SENDIASS is slightly confused. SEND20A would be where you are asking for permission to appeal because you believe SENDIST erred in law. Sometimes appeals can still be made where there is a procedural error. Sometimes people complete a SEND20C then if that isn’t successful go on to complete a SEND20A, but it depends on the circumstances of the case. SEND20B would be for a change of circumstances rather than procedural error. SEND20C would be asking for the decision to be set aside (rather than appeal). This can be for procedural irregularities. It depends on what the document was as to whether this includes your circumstances. The LA not using certain evidence doesn’t always automatically make it a procedural error. But if, for example, the LA didn’t include a witness statement from the school, it can be. Did you check the bundle? Without knowing more/seeing the case, it is hard to say for certain, but from your post, I would have tried SEND20C first. Although even with SEND20A, SENDIST will review their decision anyway before granting permission.
Time is how long is a piece of string I’m afraid. To properly hear back on the application, a lot can be up to 10 weeks, although I know some longer, but you will hear before then with an acknowledgement.
I know the forms say to post or fax, but SENDIST accepts emails.
Just so you know, late applications can also be made. Although aren’t guaranteed to be accepted.
Did you submit SARs? If not, do that now. To the school, LA and any other service/agency/professional DD involved.
If DS is autistic, that is most likely his primary need is ASD. Not all LAs state the primary need in EHCPs. Focus on overall presentation rather than just primary need. There can be a huge scope in needs within a primary need and just because a school caters for a certain type of primary need doesn’t mean they are necessarily the right school for a child.