I think my post might have said something about Polaris and I don't believe that policy applies to Polaris.
All Polaris seats are the same basic frame. The carrier either fits in the foot-fell area or it does not. The carrier does not go against the bulkhead.
Bulkhead rows in Polaris have more room under in the foot-well area; therefore, the carrier or dog is more likely to fit more commodiously than in other rows and make fore a more comfortable human and/or dog during the flight.
While practically speaking you are right - the dog will fit - the official policy is that In-Cabin pets in a carrier can't be under the footwell in ANY Polaris J seat on the 787-10, and in addition never in any bulkheads. It's stated on the website.
https://www.united.com/ual/en/us/fly.../in-cabin.html
That said, if the OP is travelling without a carrier and follows the
Service Animal / ESA process it may all work out.