I agree, Execs are usually more of a handicap than an asset and actors will always want more character motivation written in. But i have sympathy with even the most meddling of actors' desire to see a script before signing up.
Wow, that final act, though - i still can't believe they messed it up so badly - it make's SPECTRE so uneven - even for a Bond movie! Like all of Craig's Bonds there's a wonderfully melancholic, introspective quality throughout - never more so than SPECTRE's dark brooding menace of something sinister lurking on the periphery. And like CR, QoS and SF there are perkier moments, beats of wit, but those previous 3 movies begin and end on sombre, darker notes.
There's 'darkness' aplenty in SPECTREs final act - derelict HQ and faux freudian set design suggesting we're inside Bond's psyche with the red explosives wire veining its way through every synaptic corridor. Special 'subconscious' cells fill the basement with bad memories of Vesper and the villains (Green's absence did make me laugh - Eon don't want to remind us about that one) and stood at the root of it all a childhood trauma personified literally by Blofeld - no longer just a symbol to project all of Bond's trauma upon, but the actual physical cause.
It's highly ambitious stuff that can be easily botched in the telling - very easily botched, apparently.
It was all inexplicably half-baked - this subconscious metaphor never really addressed as metaphor, becoming merely a Batman & Robin-esque setting for the Riddler's dastardly plan. I can imagine the writers jotting it down broadly and explaining the elaborate metaphore in more detail to the producers, actors et al who loved the idea, but then failed in their attempt to inject this detail and nuance in the script itself.
The MI6 HQ scene culminates with Bond having frantically searched the building fo Maddy, stands atop it, beaten, sneered at by Blofeld in his chopper...only to a hear squeak from a nearby room... Oh, there she is Feels like it was written on a napkin in a bar.
It was like a CBeebie (UK children's tv) attempt at darker depths - a camp version of Spooks with the script desperately reaching in order to give the 'team' something to do. This was never a concern in previous Bond movies and seems to be a Mendes thing - after his Home Alone sequence in Skyfall Manor. Although this rot did set in with TND and DAD, when they tried to make Bond a buddy-movie with Yeoh and Berry, so maybe it's not [only] Mendes, but a combination of Producers wanting a more ensemble feel to it and Mendes desire to give the luvvies something meatier to do. But i don't really blame Mendes, it's a director's prerogative to recognise the talent's of Fiennes and Whishaw and want to film that. It's down to the producers to know what works for their brand and Spooks is not their brand.
Your description of the events behind the butchering of this final act tallies well the final results - it seems most of the blame rests upon the shoulder-pads of anxious Execs.