"IF BJ fails to keep his seat (please let this happen) who do you think would become the leader of the Tories?"
Sadly unlikely, but my thoughts - maybe totally wrong 
- :
The current Cabinet - who remain as caretaker ministers until replaced by a new PM - would choose a "temporary" leader, possibly JRM - if he keeps his own seat - since JRM is reportedly acting as BJ's deputy.
BJ's fate depends on whether the Tories have a majority and if so, what size
With a 30+ working majority, he might - and only might - be given a very safe seat, with the former occupant boosted to the Lords
Ditto if no new govt can be formed and hence there is a 2nd GE within a month - no time for a contest
Otherwise, any less than a Tory majority of 30 and imo he would be replaced
With a smaller majority, JRM would stay in place while MPs decide the new leader & PM by "coronation" without a contest,
to prevent the start of the new govt being made even more chaotic and embarassing by a contest for PM
- remember that the clock will be ticking away to 31 December 2020 and the end of transition
If other parties form a govt, then there would be a formal Tory leadership contest
Replacement for BJ would depend on whether Tory MPs have had a sufficient fright to move away from the hard right - in which case they would probably choose a rising but safe star, e.g. Tom Tugenhadt
If not, then likely JRM or even Raab < double shudder >