if he is only going to see them for 4-6 hours then it isnt that long and he will probably manage your ds3 fine - i also dont think you should tag along watching.
but i would get your social worker from chidlren with disabiltiies team informed and involved with this.
he should be comnpetent enough to dail 999 in case of a seizure - does ds take meds with him in case? is there specific training needed for that?
that is more important than signing to be honest - i think you have to let him get on with it and see how it goes. non verbal children have ways of communicating distress basic wants and needs -
more useful to send along a personal passport book - one which tells about your DS and what he likes/dislikes and which signs he uses for specific things - this is good to make for anyone meeting him or being with him.
have a look at scope version
www.scope.org.uk/help-and-information/publications/scope-communication-passport
www.scope.org.uk/sites/default/files/pdfs/Communication/Scope_communication_passport.pdf
your ex going on a singing course really is not going to make that much of a difference - it is the day to day contact that will help and that isnt going to happen.
going on a signing course helps but realsitically i dont think you can amke that a prerequisite - my son is non verbal also his carers have picked up on the way and my ds signs tend to be invidivual.
unless your ds is a very fluent BSL signer then going on a makaton or other signing course is not crucial.
how many signs does your son realsitically use and understand?
if he is like my son then very few - which all be written downa dn explained in his eprsonal passport to go with him.
(my son now uses a communciaiton device but still uses one or two signs routinely so i tell carers about them)
what can your ds communciate in sign? how many signs does he have? really - a specific to him course would be more appropriate - but maybe the other dc can interpret as they know him better anyway,
if he wont mediate then you likely to end up in court anyway if he gets stroppy with you - but if all he wants is 4-6 hours per month then i would go along with that.
also i wouldnt give him money for food -he is their dad not a paid babysitter.
if he wants you to pay - have him ask that thru his solciitor. (his solicitor should then put him right)
how old is DS 1 - old enough to also know to dial 999 if ds3 has a seizure?