Yeah, I think that's the practical solution, ultimately. Succeeding at an ADA employment suit is difficult and expensive for most individuals, so I would not be surprised if there was no ADA case in any federal court involving dogs in any way. Even filing an EEOC charge takes a good bit of effort, and that's the minimum to start down the road to an ADA employment discrimination suit.
Generally, I wouldn't expect pet dogs to be appearing at a company that already has a severely allergic employee, provided that's known information. The other way around (new employee with a service dog vs already present and highly allergic employee), I would guess comes down to the "undue hardship" test. If both cannot be accommodated, it probably is not considered reasonable to fire the existing employee in favor of the new person and their dog. Unless, of course, it is, because the company believes the new person will provide more value for the company than retaining the old person would.
That essentially means that the ambiguity is decided by private companies, which I find distasteful, but I would not be surprised if that's how it actually works out.