" An independent OpenReach would be under even more pressure to cut costs with no one with any clout fighting its corner. "
I suggest you look at what happened in New Zealand (but you probably already did and know what happened there)
That country had the opportunity to let its telco keep the BT/Openreach model or cleave it and chose the latter course.
With the dead hand of head office removed from the handbrake, Chorus is doing extremely well. ALL those decisions you cite are not dictated by Ofcom, they're dictated by BT and they closely parallel what Spark(telecom NZ) was doing to Chorus pre-separation in order to make it appear as sick as possible (including major campaigns that mades its share price drop 30% of the day of issue). The reality is that 5 years later, Spark is looking decidely ill, whilst almost everyone else is extremely happy with the quality of service they're getting (the lines company went from being unreachable to proactively dealing with customers) and the prices they're paying (dropped substantially from what the monopoly telco was charging.
New Zealand used to be a poster child for how NOT to privatise your telco, but when they finally took action, they've become an example the UK needs to look at, as when the Telco saw the writing on the wall it tried to sell the BT/Openreach model to the point of voluntarily splitting up operations to stave of government intervention. It was only after assessing BT's ongoing market abuse in the UK that NZ regulators ruled the companies must be completely split.
Openreach as a separate company - like national grid or transco - must have rules preventing majority ownership or rent seeking behaviour, but the NZ experience shows that a free-to-sell Openreach WILL sell to all-comers, actively seeking them out.
The issue of virgin's cables not reaching everywhere? Not really an issue if Openreach is leasing duct access. Digging up the road is the most expensive part of cabling. Etc.
The arguments which keep being raised by BT and its cronies/apologists to the effect that Openreach can't possibly survive if cleaved off are looking increasingly desperate, given that experience in other markets has shown this simply is not the case.