Capacitor and ISP issue
A simple solution could be to connect the junction of C5 and R1 via a 10k resistor to PB2, and connect the ISP connector directly to PB2.

This is a general solution that you can use whenever you want to override a signal with a stronger signal that will not be present (or is in tri-state) when the weaker signal must be read. Of course, the series resistor put 'before' the weaker signal to make it weak must not cause a problem. Generally, digital microcontroller input pins have a very high impedance, so a 10k extar won't be noticed at anything but very high frequencies. (but remember: analog inputs might might have a much lower impedance, so in that case this trick likely won't work.)