Those for sale are from British expats. Britain's Brexit caused this a few years ago. Nothing new. Just homes sitting vacant and which have been devaluing the property market with the saturation of excess homes.

It needs an island vent. It offsets under the window and by code you cant offset below flood level rim of the fixture. To vent without the offset would require it to rise straight up which it can't...obviously.

You don't purchase an island vent, you have to oversize the drain and loop the vent from the sink apex back into the oversize pipe, in this case, it should be a 3" drain. here is an illustration https://www.google.com/imgres?imgurl=https%3A%2F%2Fterrylove.com%2Fimages%2Fisland_vent_1b.jpg&imgrefurl=https%3A%2F%2Fterrylove.com%2Fforums%2Findex.php%3Fthreads%2Fisland-sink-venting.43049%2F&tbnid=H1dkqasDSFGZrM&vet=12ahUKEwipjuPqoar9AhURh-4BHeknA4gQMygBegUIARDLAw..i&docid=8ppTsAUDa6_OhM&w=512&h=459&q=island%20vent&client=firefox-b-1-d&ved=2ahUKEwipjuPqoar9AhURh-4BHeknA4gQMygBegUIARDLAw

Oh Boy.... You don't understand pipes are in a freeze wall?

Hey OP, you did the biggest abortion of a remodel. The other seam of the counter is equally horrid. Plus, who installs the faucet right at the trim of a window where the pipes will freeze in a cold snap.

Also, I doubt the sink drain has proper venting, which would be an island vent system.

The sell expensive copy paper and ink.

Nice spreadsheet