What if threads tend to die out all by themselves, or such is my personal opinion
As for what if, I suppose that Oath as an Oath mattered more to the ruling house than to the commoners. Thingol with the help of Eomer, Gandalf i.e., the whole lot of 'marketing managers' makes quite a PR campaign to convince his army to go. I don't intend to say they would not obey direct orders to muster and march on without speeches Thingol repeatedly inspires them on with, but it would have take a lot more of frunts and complaints. Indeed, what mere commoner knows about Gondor? A place far away, they are our friends, right, but what with wargs afoot in my own country, what the heck to go to war abroad? So riders need constantly be
reminded about their own personal allegiance to Thingol's house.
Again, I do not intend to sound as if riders would break their loyalty and refuse to go without Thingol and the inspiring, but it resembles modern PR campaign allright.
So, even if, as rightly noted by Ohtatyaro, my compliments, it were possible for Sauron to launch massive atack before its ripe time, with two opposing leaders, it may have ended up in just another civil war, granted that Thengel would stand up to his father in the first place. Two PR campaings at a clash - 1. Let us be true to our word! 2. Let us protect our own home, and to hell with Gondor!. Bet you a dollar at the outcome?
I suppose, what with US president elections tomorrow, most of our members must be familiar with the principle
cheers