If someone is lobbing Molotov cocktails from across the street, or from a plane five hundred feet overhead, or a remotely piloted drone at the end of the street, or sabotaging the gas mains from the other end of the city block? Things get iffy.
In fairness, a clever active defender is better than nothing here.
Even assuming it can't leave the apartment or influence things outside of it easily things like bursting pipes, triggering alarms, and hurling physical projectiles back at things out of range, and similar tactics are on the table for it.
It's not perfect, but it does make things harder.
Again; not our highest priority right now, but Harry's setup isn't the best that can be done or even the best that can be done with his resources.
It's worth noting that his nevernever defenses were apparently basically nothing. Leanesidhe had to plant a garden on the other side of the gauntlet from his apartment to keep things from crawling around his wards.
He's good for his age, but his wards demonstrate how grounded in the physical and immediate expressions of magical power he is.
We might be overthinking this. The best upgrade on Harry's wards would likely be a Doggie door that Mouse can use. And Harry is much more likely to agree to that then to us helping create and bind a spirit of malice for him.
He doesn't even need a full time spirit for this, or to deal with the security issues of a doggie door.
Binder, a two bit mercenary with just enough talent to fall things up he can't personally put down, manages to keep a clan of physically superhuman summons on payroll.
They're really dumb critters, which limits their utility, but they're still active defenders who can fake being human as along as they don't need to talk.
Harry should be able to summon up some trash mobs, then let us brow beat them into taking discounted rates to be guardians.
Then he could put up a ward that triggers a summon spell under certain conditions automatically and give Mouse a manual control.
It wouldn't solve all his problems, but a hit squad that can pop up to actively contend with his enemies means they can't just sit and fired his defenses to take what they're dishing out till they fold.
Edit:
[X] No, but this is even better than what you had envisaged
-[X] There was no point where this was out of control, but the opportunity wasn't apparent until after we started.
-[X]To Helen "It might be a good idea for you to regularly mention just how safe you feel with it protecting you it's not for petting, but laps up praise. Also give it a name."
Don't exalted gods like material offerings? Burning some incense for it would be a pretty classic way to make it feel appreciated and make the unit smell nice.
It also probably has a name already, and giving new ones can be dangerous for spirits since they can alter their nature.
Might be better to ask it for a use name before giving it one.