Wow! You have a lot of contingencies planned in.
I guess you are using the LiBIM as a backup means of charging your house batteries off the alternator should the dc-to-dc converter fail. You could just use an on/off switch instead of the momentary battery “emergency start” interconnect switch on the existing interconnect solenoid. The BIM adds voltage sense (insures there is a sufficient voltage differential) and it cycles on and off (something like 15 minutes on then 20 minutes off to avoid overheating the alternator), as I understand it. Not sure this functionality is necessary for an emergency backup situation (you could manually switch it on and off every 15 minutes or so). See below about concerns wrt connecting the two systems together, however.
I have had no problem starting my generator off of two paralleled Battleborn 100 amp hour batteries. I guess, if there was to be an issue, it would be the internal battery control circuit sensing excessive current draw and cutting the battery power off. Using the emergency start switch to activate the battery interconnect solenoid (or the relay in the BIM) should allow the chassis battery/alternator combo to be used to start the generator. If this were to occur in my unit, I would probably disconnect the Battleborns first, then activate the interconnect solenoid, as personally, due to the inherent voltage differences of the two battery chemistries, and the potential for very large current surges, I don’t like the idea of directly connecting the chassis/alternator circuit to the house LiFeP batteries.
I understand you are intending a manual switch supplying 12 volts to the ignition trigger of the dc-to-dc converter. I would alternatively suggest wiring it to an ignition signal, with a switch that allows you to disconnect the signal should one of your emergency backup situations require disabling the converter. Then charging off the alternator through the converter would be automatic, and you would maintain the ability to disable (not activate) the converter should some (abnormal) situation require it. On mine, an ignition signal is available in the BCC and I tapped into that signal for my dc-to-dc converter.
2012 Solera "S"
Calif SF Bay Area