I physically have 7 volumes of a light novel adaptation. Those 7 volumes cover the first two or three volumes of the light novel out of ten total. The ending of the manga was slightly modified to better fit the hard cut (the artist explicitly said so in the afterwords of the last volume).
In short: yes, publishers will absolutely give even 10 volumes for a partial adaptation. They have the resources to and obviously they are getting more than they spend in producing and distributing the adaptation or they would have stopped doing so already.
Actually, I have a conspiracy theory that all adaptations start as partial and continue covering the novel only if they become more popular than expected during the committee meeting.
But was it intended to be that way from the start, is my question.
I view it as the opposite, and more like "We'll adapt it, because even if it loses steam, we can just give it a halfway okay end and send anyone interested to the novels".
It doesn't matter in the end, sure, but it's the whole difference between this insidious, malicious move to get more people hooked to novels by sneaking them a taste of the story in comic form (which is odd, considering that you're in the business of comics), or just an unfortunate effect of a convenient exit clause that publishers can easily use to end these adaptations, like some sort of large bladed implement that swings down and cuts things.
Like the manga adaptation you have as an example: 7 volumes, covering 20-30% of the novels. How many chapters is that, taking up how many weeks/months worth of magazine publishing slot? That's a lot of time investment for what amounts to a promotion.