The previous long list of abilities was depicting the possible advantages of non-humanoid forms, and not meant to imply that all of those properties would be possessed by a single entity. My apologies for ambiguity.
We currently have a priestess, cultists, and goblins. I would expect this to increase, but yes, stuff could potentially happen.
I suggest keeping a supply of potential hosts with minds already crippled on hand. Methods to reduce the likelihood of them being detected would include focusing on poor individuals. People significantly harming themselves in pursuit of vices. And preferring immigrants.
Our main opposition is likely to be adventurers and political opponents. Both groups are known to hold many members who are willing to kill civilised species. I would expect us to typically avoid personal combat with credible opposition, but regardless, if we are facing an opponent without a humanoid host, then it is likely that we are doing so because they are equipped to battle a humanoid form.
I played, I think that it was Resident Evil IV, on the P.C., which lead to most of my fear being reserved for the interface... It was quite a while ago, but I recall the most intimidating monsters were random mooks with massive weapons.
Adding extended reach, sword-length spikes, smaller or more aggressive torso, hardened ridges, greater ability to react to things behind us, and less exposed sensory apparatus with more coverage would all degrade a human appearance.
We are embarking upon a path of control. If we are bereft of cultists then we have already failed.
Gaseous form has issues with physical interaction, while it offers some advantages, and it would be good for our physical form to offer different advantages, just because gaseous form possesses a property does not mean that possessing that property in a physical form would be redundant. And people expect living things to be animate, if we can pass ourselves off as inanimate then it offers additional opportunities, inanimate and mobile is ideal...
It is much easier to sell a pure-white horse with a spike sticking out of its forehead as a magical cure-all then a strange human with a vial of unknown liquid. To be certain, it has been done, but it will generally be more challenging...
I think at this point it might be best if the G.M. just made our new form a multiple choice question...