Hello Lars,
Thank you for your careful and very thoughtful reading of my book. You raise some very good questions! Of course, contemporary lunar science cannot yet produce definitive answers for everything, but the case for how the lunar maria were formed is pretty convincing. This is probably not the place to go into lengthy detail, but I can certainly try to address some of your questions.
We should bear in mind that the recent Hawaiian eruptions and lava flows were on a microscopic scale compared with the events that created the lunar maria. The enormous energies released in the impacts that created the huge basins at the sites of subsequent mare development would have compressed, shattered, melted and fractured the lunar crust at the impact site. Such a compromised site would have provided a preferential route for the subsequent eruption and flow of magma on to the surface, magma that had been kept molten by the effects of churning in the interior magma ocean and the decay of radioactive elements.
As Wood says, once vented onto the surface the lavas would indeed have flowed relative easily over great distances, being much less viscous than lavas on Earth (the Moon as a whole is made up of less dense material). Also the larger maria at least would have been formed not in one go, but by successive lava flows. This is obvious in the Mare Serenitatis, where the flows of different lavas are obvious even to the eye at the telescope and appear as different colours and shades.
This would all have gone on until the Moon had cooled sufficiently to prevent the eruption of lavas on such a huge scale, (although smaller forms of volcanic activity continued long afterwards).
Thank you again for your interest!
Bill