I might be splitting hairs here but if the point-end weight stays the same (in my case 280 gr.) will the extra length of a 100 grain insert cause an arrow to tune differently than one with the standard factory length?
In my experience, Yes, it will act a bit stiffer.
Because the weight is more stretched out and not as concentrated in one spot and the insert is a bit longer thus stiffening the arrow (though that effect is very slight).
Yes, in my experience the extra length of the insert will effectively "stiffen" the dynamic spine of the shaft. The two configurations will be different. Whether or not the difference is acceptable will be up to you....
The insert acts the same as a footing really, just inside the shaft rather than outside.
Yep... the same way how HIT inserts cause an arrow to stiffen up as well.