I thought #1 was perhaps Euphorbia neriifolia, but yours seems to have flowers with much longer stems -- or perhaps the length of the stems depends on whether there are leaves present. Also E. neriifolia seems to have a varying number of stem ridges -- described as 5 where such information is mentioned, but there's a photo of the woody trunk of one in Hawaii which looks like it probably has at least 8.

Definetly let it callous. I'm not familiar with this plant but I'm sure if you don't let it heal before putting it in soil the chances of it rotting go way up. Also if it is rot I would change soil or watering habits. I think most problems are from not enough light though. Direct sun helps dry the soil faster, if its not getting direct sun the soil is probably staying wet too long, causing the roots to be drenched and restricting oxygen around the roots. If its a grafted plant it probably wont survive on its own it would have to be grafted onto another plant. That I can't help with, never tried it. I'm sure someone could help though.

My advice is avoid a multistemmed maple. They don't stay the size you see in the nursery forever and once they get older, you run into big problems that will cause them to need to be removed. They're a bad idea... one of many the growers have found to make money. It started out that multi-stemmed trees were created to display ornamental bark such as with river birch. It was a bad idea for them as well but at least there was a reason for it. I think multistemmed maples originated when one day a tree was accidently cut down and sprouted back in 3 or 4 symmetical stems. Some unsuspecting customer thought it looked pretty and asked if it was for sale. Do yourself a favor.... Unless you're out west or have alkaline soil, go with red maple. It's a better tree. It's not just half red maple like autumn blaze, it's 100% red maple. And don't get a multistemmed tree. Get a tree with one straight leader without any damage or bruises with symmetrical brancing that is not potbound. That's my 2 cents. If you want a small multi-stemmed tree, buy a tree that stays small and is naturally a multistemmed tree. Serviceberry or Ironwood for example. A 50' tree with multiple stems is just a bad bad bad idea!

There are two common types of caster sockets, metal and plastic. The plastic can crack and break inside. They are extremely hard to find as repair parts. They are usually stapled in place with upholstery or narrow crown staples. You can usually substitute with a metal type, but you may need to drill a smaller hole nearby. The metal type has a collar that snaps onto the caster's stem. You can find these easily (see link below). But before you go out and buy them, try prying them out and crimping the top with a pair of pliers. Often that will work. If not, they are simply pull out and hammer in the new ones. If the caster socket itself is falling out, it has usually worn an oversize hole in the wood frame. The easiest solution is to drill a new hole nearby and reinsert the socket. An alternative is to use an epoxy paste around the socket stem. Here is a link that might be useful: caster stem replacements