It doesn't look hollow. When they're completely rotten inside the roots tend to pull away with most of the innards at the same time. I'd give the stump a scratch with a fingernail or a brush with an old toothbrush to check for clean tissue. I suspect that would be all you'd need to do but you could ...

There's an easier way! https://www.cactuspro.com/lecture/Ashin ... ge-22.html

The group has several admins and I would recommend raising concerns like this with one or more of them in the first instance.

http://cactus-de-patagonia.de/samenliste is online.

It is, yes!
https://bcss.org.uk/product/cactusworld-2023-4/

https://adblps-graines-cactus.com/commander_EN.html is updated.

A pair of the curved style of these can help to guide it upright again: crucible tongs

I travelled to Cornwall in 1999 and took the opportunity to visit what was then Westfield Cacti when Ralph was based in Kennford. He was about to move out of the area and the business was closing - it was a mess, more-or-less abandoned. I remember feeling quite sad. It's pretty amazing that he manag...

I would pick out every last crumb of peat (is it?) and all the dead material. Or blast it with a hose. I don't feel I can advise on what to do next due to lack of experience of your conditions but, in ignorance, I would want to reestablish a few roots in a pot before replanting - in the UK I'd be th...