I see a bunch of red flags
It really seems like you are ver inconsistent withy your alkalinity. It doesn't sound like you are testing very often and your alk might be swinging rapidly. You might not even see it because you don't know the numbers before and after. People test and say "my dkh is 8, that's good right" but it could have been at 4 the day before when you did a water change. You must test consistently.
Temp dropping really low or really high will stress your corals but I wouldn't worry about 73. Consistency is what you want. Get an apex and set the thermostat for your heater.
Magnesium? I haven't seen you post any numbers for magnesium. Mag, alk and calcuim go have in hand. If you have very low magnesium your alk will not rise. The day you drop magnesium you alk will skyrocket. Start testing
Phosphates? Temperature will not increase you algae like the way you say it does. More than likely the phosphates are there and might be stressing your corals out.
I would start testing consistently for alk, cal, mag and salinity. Complete frequent small water changes and see where things move to.