Just my own thoughts for a different viewpoint and approach. The growth of hair algae on the rocks scream high phosphates to me.
So here is my summary of what I feel happened in your system. Alk overdose, death of corals, nutrients spiked because of the deaths, growth of algae... then cleaned sump out, which unfortunately spiked nutrient levels even more and also removed a potential source of removal with the chaeto, so algae got worst or at least didn't get better.
That brings us to now. I feel you have a big phosphate issue. I think a slow introduction of GFO to start removal phosphates from your system and the continued water changes will get you in the right direction. I'd add 2-3 tuxedo urchins to help eradicate the algae that has already rooted itself. With continued removal via urchins and manual removal, combined with water changes and GFO use, you'll be staring at a nice clean looking tank in no time.
Now you're probably wondering why I assume a phosphate issue instead of a phosphate deficiency like Andre's theory. To me, if you see algae growth in the tank, there definitely is phosphate in that system. And to see it grow so prolifically on the rocks, tells me there is bound phosphates in the rock. So why does he read 2ppb on a Hanna meter you ask? Isn't that a deficiency? My answer, no, because of the presence of lots of algae. The reason it read 2 ppb is because most of it was uptaken directly by the algae, leaving the water sample to show hardly any phosphate concentration.
I've never used a regular bacterial dosing scheme so you could argue I have a very homogenous population of bacteria in my system. I'm going to say that has not been detrimental to my system based on the results I've had so far with it so personally, I don't put much weight into monoculture theories. I've also don't do water changes so even more reason why my bacterial population could be very homogenous.
I agree nitrate level should be raised to about 5-8 ppm and whatever way you can get it there is ideal, as long as the often associative phosphate level increase that comes with it is managed with GFO.
My phosphate level is consistently in the 0.03 ppm or less range. My last Triton test gave me a 0 ppm result for phosphate. My personal opinion is yes, phosphates are needed for certain biological processes but very little is needed and that little bit is easily added via feeding daily. So adding more as you can understand from my point-of-view seems like a bad decision. Average levels in surface waters are 0.005 ppm of phosphate. Trust me, your corals aren't starving from not having phosphates, they are starving from not having enough nitrates in general, not specifically regarding your tank Malira.
For your tank in particular, I think nipping the algae in the bud by adding tuxedo urchins and running GFO will help your system return to a more balanced nutrient level and your acros will respond with better colors and general health.
At the end of the day, this is just my opinion. I don't aim to say anybody else's opinion is not correct or invalid. Just sharing what I've observed over the years and how I see the situation that Malira has laid in front of us with my version of a viable solution. In the end, there are a million different ways to run a successful reef tank.
