Have you asked them to change the size of the toe kick down to 4" ?

No - I wonder if that's even possible? I will check.

Which still begs the question, does Bellmont really make an entire line of cabinets that cannot accommodate a cooktop/oven combination?

Are you in the 1600 series or the 1900? I have been looking at the 1900 and they have a few different options and are customizable.

We went with the Bellmont 1900 series. We could have customized the cabinet for the larger oven, but in the end we got a smaller oven (Fagor) that fits and can be installed "flush" (It is smaller, but it's big enough for us, I've seen it in person). Cabinets are being delivered 12/10 and installation starts 12/11, so I can tell you more in a few weeks!

I can't wait to see them :) One thing that disappointed me is no toe kick drawers :( Bellmont doesn't have them

Glad I wasn't interested in them!

Are there other brand ovens that would fit the hole in the
future?

It may not have any effect going forward, but Fagor seems
to be having some financial reorganizing.

Here is a link that might be useful: Fagor declares bankruptcy

I know (now) about Fagor's pblms - at least I have my oven, just hope it works since warranty service is unlikely! And yes, other ovens will fit, but with quite a bit of jiggering.