Regarding the idea for horizontal launch of large rockets from the VBP perhaps using a wheeled trolley on a guiding launch rail, I think we should hold off on that. We may need that trolley to roll up and down the keel for other things besides launching rockets.

I just realized that each individual lift cell attached to the keel is likely to weigh as much as 60kg. They may be as little as 30kg or 40kg each, depending on the final configuration chosen, but that's still quite heavy. The VBP's minimum keel length is about 60m (in fact, configurations that reduce the average balloon mass require a longer keel). This alone presents a difficulty for crews when changing lift cells. They will also require tools and pressure suits. Pressure suits can't use umbilicals over long distances, nor do umbilicals allow free motion about a mooring point, so each crew member changing a lift cell will have to carry a 20kg to 30kg self-contained suit along with them during this procedure. Even a constant-volume pressure suit exerts force against its occupant's movement due to air pressure, so it reduces the amount its occupant can carry almost as much as the additional weight. Just walking around in one long enough is good exercise.

The typical hand truck is too small to carry a folded lift cell without some sort of box. This means that while a light dolly could bring the new lift cell out to inflate and moor it, it probably couldn't bring the old one back. A heavier hand cart with a broad platform would be required. It is unlikely that such a cart could easily pass around a large rocket cradle blocking its path.

It is possible that a trolley capable of carrying a rocket cradle could serve double duty for carrying lift cells. However, it is unlikley that it could do both at the same time. Keeping rockets on the trolley prior to launch would require removing them to use the trolley for other cargoes.