Many thoughts on that:
1/ “Being available” is not an on going activity, therefor not a valid accountability - you could object NVGO
2/ A role is always available - the person filling the role may not
In your specific case, I guess the tension is to make sure that there is always someone filling the Engineer role. In this case, if you are the Lead Link, you can assign this role to multiple persons with different focus - you are still constrained by the work agreement because it is a legal document. If as a role filler you need it, then I would suggest creating a policy like “There must always be someone filling the engineer role so the service doesn't stop” (not proper I am sure but you can get the sense).
As a Facilitator though, you can't judge objections. So I would go to integration and try to explain that to the objectors to see if given that, the objection is still relevant.