I went to Lisbon a few years back and I loved it. So will you and spouse.
I was there late October and there was a lot of rain with some downpours. You might experience some to a lot of rain which might momentarily cool things off and get you wet.
Your spouse might like muggy weather better if she knew that it is supposed to be great for your skin. I have heard that and do not know it for fact. But muggy weather is uncomfortable.
When I was in Lisbon, there were four separate HoHo buses. Each one is a separate route and they are priced separately. If you bought three, you got the fourth thrown in.
I never took one because I could never make up my mind about which route I wanted to take and I regret it. Budget and time did not allow me to buy three. I thought they were a little pricey, like $25 each route.
I do not know if they are air conditioned.
They will take you to the different neighborhoods of Lisbon as well as to different attractions and again each one goes to different neighborhoods and different attractions.
If there is a heatwave, best time to sightsee will be very early morning and evening into nighttime. If you have not yet booked your hotel, maybe you can book one with a pool and other amenities for afternoons at the hotel.
EDIT to add: I did a search on HoHo buses for Lisbon. There are many options, too many for me for right now. When in Lisbon, I stopped at an outdoor vendor who was selling all sorts of things including tickets to the HoHO buses.
But when I go on line, it seems that you can pay one price for a 24 / 48 hour HoHo bus. I do. not know if this is for more than one route. It seems to be similar to other cities.
So maybe things have changed or maybe I misunderstood what the street vendor was telling me or vice versa.