Those seem like reasonable answers to me, thanks!
Where sound is concerned, if I were ruling as DM, I think I'd say that it muffles sound, but doesn't completely silence the person inside. The spell description specifically says that the spell doesn't suffocate the person inside, which suggests that the field is air-permeable, and that would suggest to me that sound waves could get through it, at least a little. But I'd probably rule that any sounds the person inside made would be a lot harder to hear.
I agree with you about the sphere not displacing stuff it manifests around. (In my mind, when the sphere comes into being it doesn't pop into existence already complete, but sort of descends from its top-most point to surround everything inside. That also explains the role of the Reflex save -- to interpose oneself in the descending sphere in order to spoil the spell. Though by that reasoning, should one even be able to create a sphere in a continuous substance like a liquid at all? Should the spell work only in air, other gasses, or vacuum?) If one can trap something inside the sphere with another medium - water, poisonous gas, lava, etc. - that opens up some interesting tactical possibilities.
I really like the idea of plunking an invisible force field in front of a dragon just as it's about to strafe with its breath weapon; going to have to try that. (Although a dragon's blindsense will probably matter.)
I think you're right about targeting a grid intersection. But still, does that mean that you only get 1 creature inside a sphere no matter the size of the sphere and the creatures' location? If you're a 20th level wizard making a 20' diameter sphere, it's conceivable that sphere could grab a lot of creatures. How do we explain it only grabbing one?
5 sounds right too, though it's kind of a shame. It would be nice to have some battlefield control even when the spell didn't catch its target.