All it would take is a stakeout in front to see when prayers start. It is not rocket science to figure out when we daven.
a) Yes, but that's an extra step he'd need to take
b) People (hopefully) would take notice, and he'd be caught on camera
c) While you would think it's not difficult to figure out when we daven, it's not as simple as you think - ask a non-Jewish colleague how many times a day do they think Jews pray - I'm not sure many of them would be able to answer.
d) If davening times were so easy to figure out, why would we need to publish them?
If we make Shuls harder targets what stops them from targeting schools which do not have any security as well...
a) what makes you think that they're not already targeting schools?
b) I don't know about you, but my kids yeshivas all have security - guards, cameras, etc.