Update on my earlier issue. I was able to create a handler function that works for my scenario, and keeps the update in the beforeSubmit. Thanks everyone for the assistance!

// handle inline edits
function getNewValue(scriptContext, fieldId) {
    const NOT_FOUND = -1;
    var record = scriptContext.newRecord;
    if (record.getFields().indexOf(fieldId) === NOT_FOUND) {
        record = scriptContext.oldRecord;
    }
    return record.getValue({ fieldId: fieldId });
}

var oldValue = scriptContext.oldRecord.getValue({ fieldId: myCustomFieldId });
var newValue = getNewValue(scriptContext, myCustomFieldId);