I have a customer in the flower shop who plants a "Dahliette" in the center of her pots every year & positively crams them with all kinds of container plants & they do just fine.

I suspect you are not getting any answers because you are looking for a definitive answer for something not easy to define. Meaning (and this is my 2 cents worth) - dahlias are both finicky and yet they are adaptive to their known environment. That is only how I can explain my garden full of flowers when we have summers where you may not see the sun for months. Granted, sun can filter thru fog anyway but my dahlias that have been in the yard a year or more tend to get better with age and have adapted to the fact that the ground is almost permanently moist from fog, the dahlias get so wet the larger ones tend to fall over and the sun (on good days) shines only about 5-6 hours. I have found that when they get a good dose of sun daily (like this spectacular year) they tend to wilt after blooming. So I have deduced they have adapted to their conditions and have learned to prosper even during 60 straight foggy days, like last year. This year, I planted 6 dahlias in the shadiest part of the yard and so far 5 have sprouted, one is almost blooming and one is MIA so far. This part of the yard gets about 4 hours of direct sun on a good day and they seem to be doing fine anyway. In a nutshell, despite what growers tell you, I think dahlias can do fine without 8 hours of sunlight. My yard in August can prove it.

IME Nepeta do well with lots of heat, lean soil and need very little water. then they stay reasonable compact but still spread. but if you combine them with Dahlias and keep those happy and flowering, the Nepeta will probably sprawl as they are completely overfed. I would wack them back now (they probably regrow quickly) to give the dahlias room and plant the tubers in another place next season. do you happen to know what kind of Nepeta species you have?

With what you have shown us …it does not look like it is feasible…with more info … a floor plan…better pictures of all the adjoining spaces… a better understanding of you needs for the office space… other ideas may develop…I agree with PPF… this will likely require a contractor… if you can find one that is qualified and willing to do more than just build walls