Those are very sensible considerations. I am also a big fan of Gel's, except for their cost and energy density as you point out. I have happily used them in many military projects over the years where safety and ruggedness were paramount.
I cannot speak for every external BMS system, but my REC BMS and I'm sure the vast majority of others, do provide over-voltage protection against charge sources regardless of whether they are controlled and/or monitored by the BMS. The BMS will disconnect the LFP battery when an over voltage is detected.
With regard to the other safety issue you mention, where a short occurs in the lead battery of a directly-connected hybrid system, is mitigated by means of the series fusing provided on both the LFP and Lead batteries. In my system for example, 400A class T fuses protect both batteries.
I don't know what the fusing arrangement is on LFP batteries with internal BMS.
It's worth noting that most (if not all) external BMS units are fail-safe. The LFP is dis-connected by default and is only connected when all parameters are within spec. In the event of a failure of the BMS, the LFP is disconnected. (Another reason that it's nice to have the lead component in the circuit to keep the lights on).