Roberton is definitely a locative surname, literally the "ton" or "town" of Robert. The etymology very distinct from Robertson. Nesbit in A System of Heraldry Vol II makes the case for Roberton going back to c1220 with charter evidence. Most academics such as GSW Barrow in The Kingdom of the Scots, and others such as AA Duncan, Lauran Toorians, Richard Oram, and more recently Alex Fleming in Scotland and the Flemish People, 2018 Birlinn Books all cite Robert the brother of Lambin Fleming as the progenitor the Roberton family. They were Roberton of that Ilk, of Earnock, of Bedlay. Completely separate to Robertson.
As for the Swedish connection, both Nesbit and Stoddart cite Matthew Roberton following the Marquis of Hamilton to Sweden (via Germany I think). They actually cite Robsalm, but I found "af Robsahm" but in either case I agree it's incongruous that the name evolved to an equivalent of Robertson.What piqued my interest was the heraldic reference where the Robsalm arms were described as "a helm on a field with a plume" topped with an anchor which very much echoes the Roberton arms "quarterly, 1st and 4th gules a close helm argent, 2nd and 3rd Argent a cross crossed fitchee gules" crest, an anchor proper. Robertson were "gules three boars heads or".
That's stating what I know based on the work of others. I'm at a loss as to what happened to the Robertons who left Scotland with the Marquis but if you're interested in collaborating, I'm keen to find out.