An 880nM long pass filter will absorb all the laser light. The wavelength used to describe most long pass filters is the wavelength at which about half of the energy is absorbed. That is why I recommended the RG715 (red glass that passes half of 715 nm, and more at longer wavelengths, like 90 % at 780 nM).
I would cement the filter right against the photo diode, so you can play around with different tubes and lenses without having to change the filter.
Just change the filter to something that passes the laser wavelength. Bits of red plastic are easy to find.
Don't know, and it is way too important a question for you to be taking a stranger's word for, anyway. You are going to be legally and morally responsible for the damage caused to anyone by your creation. And the damaged person is most likely going to be you.