Uhh the screw in that location actually reduces stress in the acrylic mid span. So no, adding a hole there wont increase the chances of a crack occurring. Pyotor has a valid point, in that the bow in the acrylic loads the sealing ring unevenly. Orings are recommended to have a optimal squish for sealing against set pressures. Loading unevenly means some areas are over squished, some is optimal and some is under squished. Also acrylic is very good at creep so yes there is a possibility of leakage at a later date due to deformation of the plastic and loss of preload in the center. Creep occurs at room temperature for loaded plastics, and heat accelerates the process.
I dunno, based on that photo, the pump attachment bolt right above it, so you'd be loading the region between those two holes in shear. There's too many variables in this particular example (especially regarding the flatness of the pump housing itself) so I can't really say with any certainty whether this additional shear in the corner is of concern.
In terms of creep, from my (very limited) experience with plastics (mainly non-structural POM and PEEK), I would be more concerned about thermally induced creep at regions of higher stresses (fastener holes/notches, and especially holes with threadforms, in this case). You are correct that on a longer timeframe, loss of preload due to creep could be a problem, I'm just not entirely convinced that it is of the same order of importance as the loss of preload due to thermally induced creep around threadforms ( I don't have as much experience with acrylics compared to POM/PEEK/Nylon).
In any case, I stand by my point about the modded hole. One way to improve that would be to actually counterbore the bottom plate and use a bolted joint with a nylon washer instead of tapping directly into acrylic (but I am not convinced there's adequate margin in the bottom plate to do so)