I think I know how the flood could have been stopped without firing the halo array.
you would need a group of volunteers who would succumb to a DNA altering procedure.
the geas would than be altered so when the flood consumed them and integrates them the geas would act like a virus causing the flood to become reckless and eventually self destruct.
since the flood acts as a hive mind if you could infect the gravemind it would be easy peezy baby, yeah.
perhaps this was the alternate plan we have heard rumblings of.
Hmm… an interesting idea but we know this plan has to deal with the Ark. So possibly the technology for this relies on it? Another thing we have to consider is the importance of the absolute record.
Wasn’t this was the Forerunners THOUGHT humanity did (without the geas, I think, but basically infecting the Flood from within)? Also the technology that is geas isn’t very well explained so I’m hesitant to apply it… well, anywhere. Hey! Maybe I’ll pop that into canon fodder…
There’s no guarantee a geas could have any effect on the flood. While the flood seems to run off a precursor mind, it doesn’t seem like other consumed minds have any say in the flood’s goals, they’re used more like processing units and data repositories. Otherwise i’m sure the collective will of all the people who DIDNT like the flood but got consumed anyway would be able to make the flood stop naturally.
Also the flood genome undoubtedly is extremely complex since it can incorporate and repurpose other vastly different species cells and organs.
perhaps this is why they were studying the flood, to see if this could work.