As most of the Scottish farmed salmon industry is now owned by the Norwegians they will be taking your money anyway. The Norwegian government has in recent years clamped down on much of the environmental damage caused by current salmon farming practices in Norway, if only the Scottish government had done the same.
Regarding jobs, you are correct that salmon farming does provide employment, however the industry has led to the decimation of wild salmon and sea trout populations and the fishing tourism industry and employment associated with it. So no positive benefit there.
As for using wrasse to remove sea lice from salmon, this is just a PR exercise, with no significant impact on populations of lice in sea cages. If it worked they would all be at it - they are not.
At present the industry is not moving away from chemicals - there is industry data available to demonstrate this.
If this was happening on land where people could see the dire consequences it would have been banned long ago.
The good news is that land based production does provide a more environmentally sound method of production - without the pollution, the sea-lice impacting on wild fish species, and the escapee fish damaging the genetic integrity of wild salmonids. It's still in the early stages but its future look promising, provided the consumer supports it when it's available. It's a free country you can buy what ever type of salmon you choose but don't delude yourself by thinking Scottish farmed salmon is sustainable in its current form.
I am not a tree hugger, I just care about our wild fish. I want to be able to head back up home in the future and be able to go out and catch a wild salmon or sea trout in a beautiful environment and decide whether I want to take it home to eat with my family or return it to fight another day.
If I do take one home to eat I might well smoke it using the excellent advice found on this thread