Are there any RAW answers to what happens when Cloudkill encounters a wall that the gas could not move through?
No. The limit of explanation is what you have quoted.
'Solid obsticle' is a relative term. Brick walls are solid but they are not gastight but assuming you do mean impervious to the gas, the DM will have to rule on what happens. Your suggestions are all fine and no doubt there are a number of others that could be made.