My DS has recently been awarded LRM. I did think about appealing it but I've decided not to in my case.
You are right in that if they look at the award again, they can change it. I think they like that to be known to put people off. How long is your award for?
The criteria for HRM is high. This from the Carer's UK website:
'The higher rate mobility component can be paid to a child from the age of three years. It is for children who are unable, or virtually unable to walk, or where the exertion required to walk would constitute a danger to their life or would be likely to lead to a serious deterioration in their health. Children can also qualify if they have a severe visual impairment, are both deaf and blind, or are severely mentally impaired.
To qualify for the higher rate mobility component because of severe mental impairment, the child has to meet all the following criteria:
- be entitled to the higher rate care component of DLA
- suffer from a state of arrested development or incomplete physical development of the brain which results in severe mental impairment of intelligence or social functioning
- exhibit ‘disruptive behaviour’ which ‘is extreme’ and ‘regularly requires another person to intervene and physically restrain them to prevent them from causing injury to themselves or another, or damage to property’
- be so unpredictable that they require another person to watch over them whenever they are awake.'
That all said. I do know children with ASD who have qualified. It's partly how you put the evidence together.
Would your DS's school be prepared to write supporting evidence? What kind of evidence did you include? It's worth including a diary of a day/week in your life really detailing the difficulties you and they face in terms of getting around.
I know what you mean about not being able to face all the forms and appeals.
The Citizens Advice offers a service to help with DLA appeals. I don't know how easy it is to get through though as I've not used them myself:
www.citizensadvice.org.uk/benefits/sick-or-disabled-people-and-carers/disability-living-allowance/dla-appeals/dla-mandatory-reconsideration/
I'm sorry you're going through this. Life can be hard enough without this extra stress 
Good luck!