Just for clarity, the L & R Front will be disconnected. There are 2 Presence outputs, Ext Speaker 1 can be assigned as Front Presence, or Zone 2 or 3. Ext Speaker 2 can be Front Bi-Amp, Rear Presence or Zone 2 or 3, but you cannot assign both Ext Speaker 1 & 2 to a single Zone. Pre-Outs are live no matter the Amp assign.
That's what I thought based on the information in the owner's manual. I have never doubted you that the FL/FR could be disconnected based on the way you described it, as it sounds totally logical to me.
The remaining question is, it looks like (again, based on the owners manual) to assign the FL/FR channels to the pre outs so as to free up those two internal amp channels to the presence or zone 2/3 channels, one would have to select their speaker layout to one of the specified speaker configurations described in the 3080's owner's manual. (Page 33-35):
7.2.4 [ext.Front], 7.2.2 [ext.Front]+1Zone and 7.2 [ext.Front]+2Zone
So if you are only running say 5.1, can you still assign FL and FR to pre out by using one of the 3 speaker layouts that all involved 7.2.x, then reconfigure the configuration back to 5.1 after disconnecting FL and FR internal amps without getting an error messages and the FL/FR amps remained disconnected? I don't have any concern either way as I run 11 channels anyway but I sure many readers running fewer channels may also want to disconnect the internal amps so they can feel good about using their external amps that may require the pre out to stay pristine at >2 V unbalanced.
Thank you.