“Now your contention is that the original Republican Party platform is the only acceptable resource for understanding the reasons behind the GOP’s founding.”
Do you have a better source?
If the first published Republican Party platform was a pretext for something that could not be publicly acknowledged, it wouldn’t be the first time people have used a false flag to advance their own economic and political best self interests.
What have you heard?
How about the proceedings from the 1856 Republican National Convention, during which the first Republican platform was debated and developed?
https://archive.org/details/proceedingsoffir00repu/page/n8/mode/1up?view=theater
I took some time to skim through the 1856 convention proceedings, and suggest you do the same. I don't think a reasonable person could read this and conclude anything other than the formative Republicans' goal was to end slavery in the USA, but chose to limit their platform to the expansion of slavery in order to garner popular support and to avoid overstepping the States' rights by demanding a federal abolition of slavery. One relevant excerpt from the proceedings:
P. 16-17, introductory speech by Robert Emmet, President of the Republican Convention: "Without exception, all the great men of that day [the founding of the republic] foresaw and predicted that slavery, although it could not be summarily and suddenly abolished, would die out in this country. All acknowledged that it was an evil. All acknowledged that it was the policy of the country gradually to get rid of it. That was the policy of that day. That policy led to the adoption of what was called the Missouri Compromise. [...] We could not make all the Southern States free at once. We had then to draw a line; and let it be understood that it was by that line - the Missouri Compromise - slavery was to be limited, and that it should never extend north of it."