I am looking for some verification of thought process. I have come across a nearly new lower level Diamonmdback 27.5" hardtail MTB. The issue is the front 100mm fork is broken. Doing some online searching for a replacement fork has OEM budget forks (SuntourXCT) for roughly half of the bikes original cost. I can however purchase a 29er fork(Rockshox xc32) with some deep discounts for less than the Suntour. I realize the longer dropout to crown length of the 29er (19 1/4" vs 17 1/4" for a 27.5) will alter the handling. If I got an 80mm 29er fork, that difference would be cut in half. My thinking is that the 1" difference in length would be similar to the difference of the fork length when set to "soft" dampening versus "firm" dampening while static.
Your analysis is correct, the difference between a 29'r fork and a 27.5 fork gets lost in the noise when you consider the variation in sag and setup as long as you stick with a 27.5 wheel.
I use a 29'r fork and 27.5" front wheel on my 26 inch bike. This is enough of a difference to significantly change the geometry of the bike, but it's a change I like.
Using a 29'r fork allows you a wide range of 27.5 tires.
As the axle is also higher, you need to really consider the Ground to Crown height rather than the axle to ground height. Also as the 29er is a larger diameter wheel, it may not clear the down tube, and may interfere with the feet when the pedals are horizontal.
Combining all this and the fact the geometry will alter the handling somewhat in ways that may or may not be significant depending on the bike and your person riding style it seems to be a fairly big risk. Only you can decide if its worth it.