Engaging in a dialogue with Watchtower Followers can be a vexing task especially if you are playing by rules detirmined by objective analysis, as distinct from the eisegetical conditioning adopted by them and which sees them reading INTO a given text a meaning they want to see.
Your proposition entails two sub sets of investigation: 1. The meaning of "Prototokos" 2. The type of Genitive that Col 1:15 falls into
1.There is no scholastic authority that will deny the fact that "Prototokos", although it may have a single meaning, also has a dual application. The word pertains to being first. It may however apply chronologically to time, or it may apply personally to rank. So one can be "Prototokos" as if one is the one born first, or it may also apply to one first in rank.
THERE IS NO DENYING THIS.
The question then is: What application did Paul have in Colossians 1:15?
Interestingly, the Watchtower did at one time admit that "Prototokos" had a meaning of "pre-eminence". Writing in the Aid book, pg 584, the writer says: "David was called the "first born" at Ps 89:27 by Jehovah due to the elevation of David to the PRE-EMINENT position in God's chosen nation". The writer, presumably Ray Franz since according to his own testimony he wrote much of the book, was honest enough to testify to this application of the word.
Later, however, when grace vanished from the Watchtower corridors of power, the Keepers of the Flame, the Custodians of Temporal Doctrinal Probity, saw fit to alter this and a revised version appeared in the Insight book, Vol 1 pg 836, the offending word "Pre-eminent" was mysteriously excised from the text. The sanitized version, conforming to current Watchtower doctrine now reads: "It seems that Jehovah was prophetically referring to the one foreshadowed by David, God's FIRSTBORN Son in heaven"
I could cite: Louw-Nida, BADG, Kittel, Abbot-Smith, Vine, Liddell and Scott, Renn, AT Robinson, Mounce, and others who will testify to the multiple application of "Prototokos". Refusing to accept such overwhelming support from these scholars especially in view of the fact that the Watchtower itself quotes them, smacks of intelectual dishonesty.
2. Is Col 1:15 a partitive genitive construction? That is a matter of debate and the jury is still out. To assume that it is, without allowing for other considerations, is to create a theology of convenience which is unworthy of sound biblical exegesis. It is possible to construe it to be partitive, but there are other possibilities. All we know for certain is that it is a genitive construction, the type of genitive [and there are several] is uncertain.
An examination of those texts offered up to support a partitive genitive, taken from the LXX is far from conclusive, and their congruence with Col 1:15 in most aspects simply does not jell. When we consider those texts which tell us of Genitive partitive constructions following "Prototokos" in the LXX, we find that there are certain elements found in the LXX, and which in fact detirmine the partitive, which are missing in Col 1:15. In the LXX, the partitive construction following "Prototokos" always shows that:
Prototokos is generic, not personal
Plural, and not singular
They are always modified by some sort of personal pronoun, or possessive noun. For instance, among the several texts offered up by Watchtower apologists to show a parallel to Col 1:15, are:
Ex 13:15 - However : The "firstborn" here are generic, not referring to any one in particular, and is constructed as a plural and is modified by "of MY sons". Thus there is no congruence with Col 1:15.
Ex 34:20 - The "firstborn" are again generic and plural, modified by "YOUR sons"
Num 3:40 - The "firstborn' is again generic and plural modified by "OF THE Sons of Israel"
The factors that indeed define the entrenchment of the partitive genitive are missing in Col 1:15, making the case for that text also being partitive weak. Wallace suggests a genitive of SUBORDINATION prompting the translation of "Firstborn OVER all creation" [NIV]
For a detailed consideration of the factors involved in this text I can recommend "Putting Jesus In His Place" by Bowman and Komoszewski.