I've looked at your query here for over 6 months, and have looked at Catherine and Jacob even before that.Here's what I don't know about either of them:
On 12 June 1762, Jacob Mechling and Catherine Hittler served as baptism sponsors at Jerusalem Western Salisbury Church.JWSC was the "home" church of Georg Michael Hittel and it would be easy to assume that this Catherine Hittle was his daughter.However, if that is the case, it causes problems in other situations.
The will of GMH states that his daughter Catherine's husband is George Knappenberger.This cannot be a mistake in that Knappenberger is also named in that will as one of the co-executors of the will.For that reason, I believe the 1762 baptism is one example of how that the families of Georg Michael Hittel and Johannes Peter Hittel were extremely aware of each others' presence in old Northampton County.
At the time of Johannes Peter's death in 1764, what is now Montgomery County still belonged to old Philadelphia County. That remained that way until after the Revolution when Montgomery County was formed in 1784.Since at least 2 of Peter's sons are found in Philadelphia County tax lists as well as Revolutionary service, there is a good chance that Peter's land holdings may have been in Philadelphia County even before his holdings in old Northampton (now Lehigh, Upper Milford township).
With that as a background, I am not sure of who the Jacob Mechling was who served with Catherine Hittler as a baptism sponsor in 1762.As you've suggested, it's not hard to find gedcom information which suggests that Catherine and Jacob later married, and then moved west.However, as much as I've looked, I still have not found documentation that supports the gedcom rendering of their story.
I'm saying that not to suggest that I doubt the gedcom, but to suggest that there is an awful lot about this family I would love to know, but simply do not at this point.
I would seriously doubt that they only had a few children.However, I also do not yet have access to any will or probate records for either Catherine or Jacob.
If December 8, 1770 (if I remember correctly, this is a date from a baptismal record from the Old Zionsville church) is correct for Jacob Jr.'s birth, it would suggest that he likely was not Jacob and Catherine's eldest child.However, even with that being said, I don't have any evidence suggesting (much less declaring) otherwise.
It does suggest that sometime between 1762 and 1770, Catherine and Jacob were married.
That's a long way from answering your question, but without specific evidence, it would not surprise to find that your question is well founded.
Here's hoping your question is answered with documentation before much longer.