Based on the information here at Acepedia, it would appear that the nation of Amber is located south of San Salvacion (because of the proximity to the "Amber" Mountains) which would put the nation of Ugellas south of Amber. Thus it would follow that Ugellas and Erusea had a maritime border in Gunther Bay while the Gunther Region still belonged to Erusea (which it did according to the AAW 2019 map at the outbreak of the Continental War before Erusea had to relinquish the Gunther Region to the IUN as a trust territory).
Regarding the nation of Delarus there is little information. There is a mention of a "border dispute" (with Erusea?) and that Delarus is supposedly "a small country" in northern Usea. Do we have more information and is Delarus really "small" in comparison to its Usean neighbors? (I have to ask because that same claim was made about San Salvacion which actually qualifies as a medium-sized or average-sized nation on the Usean continent, according to the official borderlines).
If Delarus were the "big" nation northeast of San Salvacion, it would follow that the border dispute (with Erusea) involved a maritime border and interestingly the two islands north of Erusea are not colored in Erusean red in the AC 7 OBC News broadcast (!), suggesting these belong to somebody else (curiously the same applies to Twinkle Islands and the Megalith facility which seem to be another territory Erusea had to relinquish after the Continental War perhaps).
If the information that Delarus is a "small" northern Usean Country is correct, this would only leave the two nations north of the Altoora Desert and Faith Park as possible candidates, but would make it unlikely that either of the two colonized the islands north of Erusea (because the two bigger nations to the northwest or San Salvacion would probably have done that first in history as they are much, much closer to these Islands north of Erusea).