You will have more then a handfull of work to do. When I get this right; age 1.5 years, brain damaged due to nearly drowning, extreme extension spasm, no contact.
the first thing you should work on is contact because everything stands or falls with this. Otherwise you will never know which tactic does work. So it might be smart to work on sensoric imput (fysical contact!) like stroking massage etc. How does the child respond? Positive/ negative does it depend on where you touch? (most likely) Since it apperently has a strong extensor spasm you should try to avoid direct contact with the back, soles of the feet. But try it out because in that way you will learn. When you say positioning has brought the tone down. Which position are you talking about? I suppose more or less a sitting position, flexed hips and knees?
You have to realise that the damage has come at an age where the brains are still developing and so you have to consider the age of the child (1.5 years of age) as well as the set back due to the trauma. Look at the problem as it is cerebral palsy( at least it will give you some direction) and start from there, also how many hours a day are you treating the child? (I say delibrately hours a day, since I know you will not have the time) So you need to find some one you will train to do exercises with the child otherwise there is no point of having any input at all. Sinc ethe mother seems to have the best contact with the child the choice is obvious.
After my experience sound can have an extreme effect (like a firecracker exploding when you are unaware) I suppose the brain is in total chaos and small disturbances will provoke a massive respons (extension spasm!) But first of all Get contact!
Best of luck and realise it is worth it (not alike these stupid advertisements)