I suppose they'd have to deal with whether it's shut up because Perez and Meg are off honeymooning in Capri, or have moved off the island altogether, or otherwise, plausibly, Tosh and co would keep running into Perez on the street or in the pub!
Ruth's house, while lovely, looks like it's miles from Lerwick, which I would have said would be a snag with sudden call outs...?
I think Ruth's rudeness works dramatically -- she and Tosh get on well, with Tosh being the gentler, more community-minded one and Ruth bringing in the hard edge from her Met experience, so while yes, she was definitely rude to Cora, who looked deeply unimpressed, it probably makes sense in dramatic terms to have contrasting leads.
Like the only reason Farida ended up confessing to Tosh about the death of her son last season was because her younger child was around the same age as Tosh and Donnie's daughter, and she certainly knew Donnie from the local parents' circuit. She's very embedded. Whereas Ruth's connections are more problematic (sleeping with a suspect who was a teenage boyfriend of hers and a ne'er do well, discovering stuff about her father's past philandering etc).