1) How can you force any private company to sell what they own?
A Government could force this, if it felt that the progress of the country was more important than the impoverishment of one private company.
The alternative would seem to be for the taxpayer to pay out to duplicate all the ducting and build it all over again a second time.
2) Who would buy the combined BT and Virgin infrastructure, the government?
The taxpayer. Appalling value to buy it back again since the last mile is even more ancient now than it was then since it hasn't had any investment since then, however, but we can't go backwards and reverse the disastrous decision to privatise it which has held broadband back for so long, as we don't have a time machine.
3) Who could afford to buy the combined BT and Virgin infrastructure?
The taxpayer. Not the same for BTO and VM; VM isn't a monpoly in half the country and exists in a marketplace.
4) Should this even be possible, who would still be the biggest customers of this company. BT and Virgin..... (re your influence issue)
The size of the customer has no relevance. That's the point. One of the BDUK objectives is said to be "to level the playng field". That can't happen until BT OR is taken away from BT.
In terms of who then succeeds, the quality might matter then, though. Would give the best players the chance to shine on an equal footing regardless of their size.