You need to suggest that your DH and his exW engage in mediation to iron out these issues.
The unilateral signing up for activities that the other party is then obliged to bring the children to/ from is not on at all. All activities that will take place during the other parent's parenting time need to be discussed beforehand, the cost shared, and each party's transport and kitting out obligations agreed to.
I had a clause in my visitation/ custody agreement that addressed this after exH unilaterally signed one of the DCs up for a sport camp that required me to provide a taxi service.
Dictating bedtimes - hmm.
There are many nuances to this.
Going against one parent's wishes and the children's usual routine just because you can or in a spirit of 'one in the eye for her (ha!)' is not on.
Letting children stay up way beyond a normal bedtime just to play Disney dad or mum is not on.
Insisting another household follow the 7pm rule just to be controlling is not on either. There needs to be flexibility within reason (7-8pm would be acceptable, but no later than 8).
If there is a genuine dispute about what bedtime is best for the children, there needs to be a way to discuss this.
Same goes for diet - don't be a dick goes both ways.
Again - mediation is the way to go here.
Disparaging the other parent and/ or step parent - HUGE NO.
If this happens, it needs to stop.
Mediation is needed here - and any parent engaging in disparaging of the other needs to be reminded that the family courts take parental alienation very seriously. I insisted on having a clause in my visitation / custody agreement specifically dealing with this. My exH was not to speak ill of me either to the DCs or to others in their presence/ earshot.
Forbidding grandparents access - unless this works both ways, or unless a grandparent is unfit to be around the children for whatever reason, I can't see this getting any kind of support from a mediator.
Essentially, your H and his exW need to sit down and hash all of this out, and an experienced mediator will identify personality issues or undercurrents and help both parties to agree on what is best for the children - and please bear in mind that this won't necessarily mean what makes your household run smoother.
There may have to be some compromise on your part, but please go into mediation in a spirit of good faith, keeping hurt and frustration well under control.
There's no legislation here that you can fall back on. We are talking about the ability of two adults to behave themselves even though they hate each other/ parted ways for excellent reasons, with the best interests of the children in mind.
The court has decided that both are fit parents, so that will be the starting point of mediation, and it will be necessary for each party to accept and acknowledge the fitness of the other. An adversarial stance is not the way to approach this. The welfare of the children and the strength of their relationship with both of their parents has to come first, always.
I can sense your frustration here - your family unit is suffering from the intrusion. My strong advice to you is not to fan flames of discord, to encourage collaboration and good faith communication. Please urge your H in the strongest terms to pursue mediation.