To be sure to fully understand you’re idea, if the bunker receives 2 hits by SBR, if 2 INF are within both are now defending at 3, isn’t?
Yes, but if the bunker gets a 4th hit, only 2 units fire at 4. If a 5th hit, only 1. 6th hit none.
And if there is a third Inf, it still defends @4?
Yes as long as the bunker does not have 4 or more damage points.
And two more hits by SBR will give to those 3 Inf: Def 2/2/3?
4 hits total will allow only 2 units to fire at 4.
And finally, a total of 6 hits by SBR, no more bunker: 3 Inf defending as usual?
yes. and the cost of repair is 1 IPC per damage point, just like global 40 rules.