Oh River, believe me I know from personal experience, well, that of my partner, just how much contact blocking some RPs can get away with. We had years of it. Unfortunately though, we found that the old adage of not being able to reason with someone who's unreasonable is very true .... time after time my partner (and therefore me as well because it usually affected me too) would 'compromise' (i.e. 'give in') for his ex in the hope she'd become more considerate and co-operative in the future - only to find that that other old adage of 'give an inch, take a mile' was also very apt. We accommodated lots of very short notice contact - which was only ever offered up when it suited the ex - because we too feared the children being told their dad didn't want them, and also because with all the ad hoc contact blocking we never knew when another opportunity would arise ..... yet once his ex had had her weekend away or whatever she'd go straight back to unnecessary game playing and obstructing contact on a whim. In retrospect, I actually think that 'giving in', 'stepping up' or 'compromise' - whatever you want to call it - actually had the opposite effect we'd hoped for in as much as DP's ex knew she get could away with thoughtless, selfish and rude behaviour because no-one ever said (or rarely said) NO.
And we had lots of experience of the ex seemingly being 'nice' all of a sudden - by which I mean she was behaving normally for a change instead of selfishly - because she wanted something, and you'd hope she'd finally seen the light so to speak, and then she'd just revert to being foul. It didn't matter that she was contradicting herself, going back on what was agreed two weeks prior or that she was being hypocritical. The only thing that mattered was her getting what she wanted - regardless of any affect on us, and certainly regardless of what was best for the children.
Of course I appreciate that is MY story and it'd be great if this particular ex was mature enough to recognise that mutual co-operation and honest and fair communication was the way forward. However - although it's hardly a proven science - years and years of dealing with a difficult ex, and speaking to lots of other women with similar issues - has given me a sort of 6th sense about these things .... this particular ex seems 'the type' sorry to say and I doubt she'd take a good hard look at her own attitude even if OP's DP was able to sort this out. Someone who's silly enough (at best) or selfish enough (at worst) to see no problem in booking a 2 week holiday without ensuring reliable childcare was in place first is probably not going to have a road to Damascus moment any time soon.
And again, from personal experience, I'd recommend anyone experiencing contact issues to get the legal ball rolling sooner rather than later, after giving reasonable opportunity for the RP to be fair about it. My DP waited to do this for years in the hope of appealing to her better nature, hoping she'd 'grow out of' being so spiteful etc., but in the meantime, the damage to his relationship with his kids (as a result of obstructed contact and lies surrounding this) just grew and he ultimately ended up having to sort out a much larger problem when he finally went to court than if he'd attempted to nip it in the bud from the start.
Sorry for a bit of a hijack there .......