Boy, you've got me stumped... you really do! All I can tell you is that the plant is basically toast which you know already... The only thing I'd like to point out is your watering... I know, reading through every week of your diary, you were concerned that you might overwater ... but I think you were TOO worried about that. Overwatering is a state of always being wet... never having the soil dry... and overwatering is more of a concern when the plants are really young - from seedling to veg... I noticed also that you were on a strict schedule as to when you watered and what day you gave nutes... A better option would be to let the plant tell you when it needs something... from veg on through harvest, get used to juding when the pot is dry by picking it up ... if it's super lightweight, it needs something... you can actually get to tell when there's just a little bit of water in there and even WHERE that water might be... but that's beside the point. If the pot is heavy, don't water... if the pot is light, water... And also most people will follow a schedule of water/feed/water/feed or water/water/feed/water/water... so you MAY be feeding on Sundays - but you might also be feeding on Wednesday or any other day of the week.
The other thing about watering is that when you DO give it just plain water, you need to water it enough so that there is water running out from the bottom. I always have a saucer under my pot and when water runs out, I let it sit in the saucer for about 15-30 minutes to see if the soil soaks it up from the saucer - and it usually does, especially if I've let the plant go a day too long - the soil gets so dry, it can't soak the water up immediately and it runs out. I water mine until there is standing water left in the saucer... if the plant soaks it up, I give it more water, etc. After 30 minutes or so, if there's still water in the saucer, I dump it out because you don't want your plant to have "wet feet" which could lead to root rot. What having this kind of runoff does is helps the soil clean out the excess nutrients and the salts that might be building up in it which could cause things like a calcium lockout... meaning your plant just can't absorb the calcium even if you're giving it calmag..
I have ZERO explanation for why your plant died so quickly or why your soil seemed not to absorb water... everything I've read about that condition just seems not to apply... I'm sorry your grow ended up this way - but don't give up!
Good luck