Tall, sick, adopted Dumortieri

[patternlanguage]

Good evening from the frigid midwest...
I work at an art museum that recently was about to throw out a rather large cactus that they said was bound to die. I think it used to be in someone's office, and then it was in an area with indirect light for about 6 months. Probably was not watered consistently since July.
I'm told it is a Stenecereus Dumortieri. It's about 6 feet tall.
I'd love to nurse it back to health – I'll do whatever it takes to save this beauty.
From my estimation it has pretty sever mechanical damage about 3 feet up from the base. The base looks withered from a distance, but up close the base is firm and the top ridges quite soft. There's definitely some mealy bugs – little white cottony bits appear at some of the needle clusters. All the needles look a bit "dead"...not sure if that's a thing.

I'd love to know what you all think is wrong, according to these photos – but also what I can do.
It's currently sitting about 18 feet away from a large glass block window. The room is about 65 degress and maybe gets down to 55 at night.

Thanks!

mealy bugs? 'dead' looking needles.

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[hoteidoc]

Relooked @ this - Most of base is just corking from age! What you asked if mealy's, looking like fungal, so not sure if some superficial, localized rot had/has occurred. Poke it with a toothpick or nail & see if squishy or soft. If yes, might have to perform some (hopefully) small resection of affected tissue. You do have to cut to good tissue, so it might end up being aggressive. Good news is 95% of stem looks good & then you just have a reroot issue.

[patternlanguage]

Thanks - that's encouraging.
So I guess what I'm really wondering now is. Is there anything I should do? Not knowing how it's been watered in the last 4-6 months, how do I know if it needs water? Do I not water it at all until spring? I've only cared for much smaller cacti in the past.
If you look at the 3rd photo down, those soft spots around the ridges are accelerating and also becoming hard along the ridge... rot?
Hoteidoc, I'm not sure I follow your resectioning advice... is that advanced?

[hoteidoc]

If any of those soft spots are "squishy" then there is rot in there & depending on how extensive, you might be lucky & only have to cut out a small section, then dust with cinnamon or sulfur powder. But if it's extensive, then you might have to cut all the way thru & then proceed upward until you hit healthy (green only) tissue. Then dry, which could actually be fine letting go until Spring before you put in "soil". If the soft spots are just kind of rubbery, then maybe it's just getting dehydrated from lack of H20 -- but nothing serious. When is the last time you (or know it was) H20'ed? Big plant like that can go ALL Winter w/o any serious consequences. Just didn't want you cutting stem of plant off if it was only getting dehydrated