O2 deficit will usually not show up until the end, if then. Sats 95%+ and stable are generally not an issue. When her CO2 starts to rise, that will signify a problem. That day can be postponed with good BiPAP settings because until the end, the BiPAP can reduce the effort required to exhale, which is what expels the excess CO2. As an example, that is why anyone with EPAP set above the minimum (3 or 4) who does not have pre-existing sleep apnea or airway disease should question it or try reducing it.
It's not a linear relationship among these variables and FVC (actually, SVC means more), because which muscles can still work, which ones can take over, etc. vary so much in ALS. Also, a major factor in the best BiPAP settings is how much volume the muscles can support, which is not a one-size proposition. If you overstuff them, CO2 will begin accumulating.
Many here, including my husband, lived for a year or more at the kind of FVC you are describing.