Oh and as for the ferry, Condor wanted to stay but couldn't get the council to fit the bill for at least some of the upgrade works, something which should have been thought about when they did the repairs in the early 2010's (when condor had to leave temporarily).
Remember that whilst Weymouth does not necessarily have the road connections Poole/Hamworthy has it's a much easier port to get out of and is closer to Guernsey/Jersey and hence a faster journey time, plus Weymouth has holiday traffic right on the doorstep and was easily walkable to from most Hotels in Weymouth.
Leaving Hamworthy you end up spending a fair bit of time just getting out of Poole harbour with it's speed limits and I would guess that has an impact on fuel consumption, primarily as from what I can gather they are at their most efficient doing full whack.
Naturally leaving Weymouth you are basically out of the harbour in a couple of minutes, and as such can get up to full speed quite quickly. Plus you can walk to the ship from the waiting area, no connecting bus required.
As for why they went for a bigger ship, I believe that was partially their plan all along (which would have more than likely been a bigger incat) so as to have less sailing per day as thus saving money. In the end the trimaran was purchased as, Austal had already built it, had no takers (companies preferring tried and tested catamarans), plus I believe there were links to the banks that had stakes in Condor, so they were basically forced to buy the Liberation to put money into Austal, this ended up being unpopular with a lot of passengers as whilst it can supposedly operate in higher seas than a catamaran it moves differently in the water supposedly making it more nauseating, plus reliability early on was dreadful.