so as mentioned in my post there is no extra layer if you are on a single ticket. Conformance/positive boarding only applies to o/d, if you are connecting the airline simply decides if they feel you’ll make your connection or not and can offload you/put you on the next flight if they feel you can’t which they can do at any airport. If so your bp will still show as invalid at security but that can be the same at any airport with the basic technology to update bp validity and there are many.

linking tickets means next to nothing and does not offer any protection across separate tickets with AA, ba or anyone else. So only flights booked as one itinerary would offer such protection. Note that whilst plans change, any airline will allow you to change -say - a jfk-lhr-jfk into a jfk-cdg-jfk if you want (via lhr or not). That’s not am issue at all, but of course any change will come with its own cost and recalculation and some tickets can even be non changeable (eg basic). So plans change, and airlines allow for that bit of course flexibility has a cost - as has protection. You can’t really have it both ways.

in terms of warning, I don’t remember about as as I very rarely use them out of lhr, but na absolutely indicates on their bps “you must be at security 35 minutes before your flight time” just as they mention luggage check tile limit and gate closing time. I certainly think all airlines should do that.