Disclaimer: I have no formal education in Koine.
In Robertson's Word Pictures, Robertson seems to say that the presence of "ei" (IE: "if") suggests that this is a fourth class condition indicating that the Bereans were skeptical, not just doing due diligence:
...Whether these things were so (ei ecoi tauta outw). Literally, "if
these things had it thus." The present optative in the indirect
question represents an original present indicative as in Luke 1:29
(Robertson, Grammar, pp. 1043f.). This use of ei with the optative may
be looked at as the condition of the fourth class (undetermined with
less likelihood of determination) as in Acts 17:27 ; Acts 20:16 ; Acts
24:19 ; Acts 27:12 (Robertson, Grammar, p. 1021).
He asks us to compare several other Lukan examples in Acts to get a sense of Luke's use of the optative. In Acts 17:27 the optative could be replaced with "and feel after him and find him" but the phrasing introduces the doubt, "might" feel after him, etc.:
Act 17:27 That they should seek the Lord, if haply they might
feel after him, and find him, though he be not far from every one of
Acts 20:16 For Paul had determined to sail by Ephesus, because he
would not spend the time in Asia: for he hasted, if it were
possible for him, to be at Jerusalem the day of Pentecost.
Acts 24:19 Who ought to have been here before thee, and object, if
they had ought against me.
Acts 27:12 And because the haven was not commodious to winter in, the
more part advised to depart thence also, if by any means they might
attain to Phenice, and there to winter; which is an haven of Crete,
and lieth toward the south west and north west.
In each case they are sure of their intent but less sure of the outcome of their efforts. So as I understand the verse in question it indicates that the Bereans were looking to see if the things Paul claimed were so but they were not sure going in what the outcome of their search would be. Luke says that in this way they demonstrated that they were not overly credulous nor hoping to find error. This is the healthy balance desirable in a hearer of the gospel. https://www.jstor.org/stable/2375233?seq=1#page_scan_tab_contents