You are calculating the internal volume perfectly.
As far as port noise goes, remember that graph is frequency and power based. IE if you are reading a 25m/s port noise, you can see at what frequency and power output. If it is at say 20hz at 800w, then you will get that port noise only under those conditions. Furthermore, once you add in a high pass (typically 2nd order at 20hz), this will also affect that. I’ll have to look at mine later, but I think the port noise was actually in the 20’s (at a narrow bad of frequencies) at full power. I have never heard chuffing from it.
I mocked up something similar with the mx15 and it appears that there would be no issues with it. It does have a relatively low first port resonance at 171hz. I would like to see it closer to 240. But a first port resonance in that range is not abnormal. I am getting a longer port length, but it is possible I didn’t input some data correctly (I’m on my laptop without my saved files). I was using three 7″ by 1.5″ with a 4.5 cubic foot box getting 39in long.
*Edit I just saw you wrote 1.25 not 1.5. Whoops. With a high pass, that looks completely acceptable. Your first port resonance look great at 208hz and your port velocity only goes over 17m/s at full power between 16-26hz. I don’t see any issues there. In fact it looks like a great design.