On each 4-fuse block, 1st open neutral could create an MWBC, the 2nd open neutral could create Line-Line Voltages
No. The neutral is just not connected in that particular circuit, so no current flows. It is just two wires in that circuit, no MWBC.
The reason fused neutrals were written out of code was because "the light is off so the circuit is dead, safe to work on" or "I used a meter between the hot and neutral wire, no voltage measured, safe to work on", as the person holds the still hot wire in their hands standing on the aluminum ladder sitting in water on concrete floor.... or leaning against a porcelain sink/tub with chips.
Dual fuse plugs were common back in the 60s to add to equipment. Hmm, still are on Christmas Lights!
Two of those pictured fuses are OLD. They haven't made those black ones for 60+ years.