well what about this you include all the criteria as you stated
it had to see combat operations (first combat operation)
it had to be in squadron strength (first operational or operational testing squadron)
on production airframes. (date first production model completed and/or delivered)
add first confirmed engagement with enemy and/or first kill and/or death in engagement with enemy
(whichever is chosen)
average those dates so that all the situational circumstances and everyones POV is taken into account in equal measure.
that would bring the Meteor and Volksjager into justification to be included in the set, although i am not sure about the do355, which i am not advocating for, i am just pointing out that as you said just looking at things one way or another can be pointed out to be "unfair" to one side or the other date wise. better IMO to take all sides POV into account when selecting dates and remove circumstances and policies from the equation and focus on the actual involvement of the airframes ...
In combat at squadron strength on production airframes is not unfair to either side. It can only be seen as unfair if you're trying to sneak in prototypes or aircraft that never reached squadron strength on production airframes, such as the Do335.