IPSEA (stands for Independent Provider of Special Education Advice) and SOSSEN (as in SOS (the distress signal) and SEN) are charities who support parents of DC with SEN.
EHCNA = Educational, Health and Care Needs Assessment. The needs assessment you first request as part of the EHCP process.
If an ECHP is based on needs through a needs assessment, how are those needs assessed without a diagnosis
A diagnosis isn’t required because it isn’t based on a diagnosis. It is needs led. Assessing needs doesn’t require a diagnosis and DC with the same diagnosis can have differing needs. For example, two DC with autism can have very different needs, one could be a sensory seeker and one could be sensory avoidant. An EHCNA includes gathering information from various sources e.g. you, the school, an ed psych. And can also include things like SALT and OT assessments. For the EHCNA, these focus on the needs of the child, the provision they require to meet those needs and the subsequent outcomes rather than a diagnosis. The information sought during the EHCNA then informs whether an EHCP is issued and if it is the content of it. A diagnosis alone won’t get you an EHCP and won’t ensure all the provision reasonably required is included in an EHCP.
how does the SEN register even identify children with SEN without at least a suspected diagnosis?
The legal definition of SEN does not require a diagnosis. You can see the legal definition and explanation here. Understanding the definition will help you understand why a diagnosis isn’t required and how that works. Just like being classed as disabled and reasonable adjustments under the Equality Act do not require a diagnosis. At a school level the needs are identified and supported via the Assess, Plan, Do, Review process (schools call this different things).
If an EHCP identifies those "who need more support than is typically available ...", is there another way of justifying support that is easy to put in place but just as necessary (I'm thinking ear defenders, extra time rather than Special School, SEND-Teaching Assistant or whatever)?
Some DC don’t require and EHCP and will receive support at a school SEN support level. Ear defenders could be provided at a school SEN support level. Extra time is governed by its own rules and does not require an EHCP. Because of the current problems in schools and TA is unlikely long term without an EHCP.
I realise a diagnosis won’t automatically qualify for extra time, as that will depend on the diagnosis
No, it does not depend on the diagnosis. A diagnosis alone will not qualify DD for extra time. She will have to meet the criteria for extra time regardless of any potential diagnosis.
My other option is no diagnosis, no EHCP -> no extra time -> failing a bright child.
This just isn’t the case because support in school and EHCPs are based on needs, not diagnosis. The school must make their best endeavours to meet a pupil’s SEN regardless of whether they have a diagnosis or not. An EHCP does not require a diagnosis. Extra time does not require a diagnosis.