Hi OP, please forgive me I haven't read the full thread but did read some. I've been where you are. My ds has severe autism and was diagnosed at two. It should have been earlier but (all the usual nhs/ la excuses).
You're right go mourn, it's awful and the realisation and fear of the future is relentless. You have all these ideas of what the future holds and then clearly its never going to happen.
Apply for the ehcp now, yourself. When I scrolled through I saw mention of sendiass, please take their advice with a pinch of salt. Although they should be impartial, usually they aren't as they are a commissioned service of the LA. Phone sossen or ipsea as these will give you good advice based on law not what the LA have put into place (all LA's are over budget on sen spending, your child will cost long term so be prepared for delays and excuses even when unlawful). The threshold for an ehcp assessment is low, if thd LA refuse then put in for appeal to tribunal.
Your child needs an OT assessment by an OT trained in sensory integration. They need a speech therapy assessment by a speech therapist trained in aac (both high and low tech) and autism and an educational Psychologist report. These should be done as a minimum as part of the ehcp assessment. Of the la doesn't have means to do them within the assessment timeframes then they should pay for private reports to be commissioned (this is a good thing if they do as it's likely the reports will be of better quality). It would be reasonable for you to ask for any other assessments you feel your child needs to fully detail their needs and the provision to meet them eg dietitian, sleep practitioner, social care, la OT to look at adjustments to the home.
You need to start with very basic communication with your child in a way that they'll be able to engage. Think pictures, signs, vocal words all at the same time so they get lots of re-enforcement and the best chance to communicate. Use a very basic now and next/ visual timetables strip so they start to learn what is/ going to happen/ing to them. You could also use objects of reference eg at night time tje same Teddy bear, hold it to them and say "bed time" then put them to bed or use one sippy cup for "drink".
Pecs (picture exchange system) didn't work for my son but he loved the ipad and could use apps. So we started a hi tech comms app (there are lots to choose from, we went with words for life, they get discounted around autism awareness day but you should be able to get if funded via the ehcp if the slt says they need it- push for a hi tech aac trial- your child isn't too young and revisit if it doesn't work). These apps basically become your child's voice but they don't replace their voice the evidence is that it supports and in many cases leads to verbal comms (it did with my child eventually). They press a button and it vocalises, start by finding something motivating maybe spinning on a chair if they like thst and using "stop" and "Go" or "more" for their favourite sweet. Model it yourself then when they follow or even try to do it react to them.
Toileting maybe an issue. You should be able to get free nappies/ pull ups from the nhs. Don't give up on toilet training, my child came out of nappies at ten and is still learning aspects but they are making progress. Ot will be vital for this.
Any and all progress is worth celebrating.
The home adaptations come through a grant that the la apply for. It could be for a wet room if toileting is an issue long term or a changing bed to help your back, it could be fencing to make your garden safe for burning off energy, it could be safety doors so you can lock the kitchen, it could be alarms so you know when they get out of bed (to wake you), it could be looking at your windows to make sure they're safe.
Look up Facebook groups for patents of children eith severe autism. You may find some of the general autism groups font have a clue.
Look at schools now and get an idea of where you want them to go. When you know discuss with sossen why and what to do.
Your local parent carer group may have other parents of children with similar needs. It's always good to make these friendships if you can, they'll know how things are locally and may have already been there and done that. It's also good to have people to talk to who get it.
Apply for dla and a blue badge. Apply to family fund if you meet the criteria and cauldwell trust for a sensory kit if you meet their criteria.
Pick your battles, cry if you need to, it's a long road. Do not take nhs or LA'S word as gospel.