Afaik we cannot still confirm if satoshi is a single person or an entity for a group of person. So we cannot categorically answer your questions. But I think to create a software this complex, you will need some help either on coding, math, or implementation.
The current belief is that satoshi nakamoto is a university researcher Dr Craig Wright. This means that he would have recieved help from fellow professors and could always have done it without them noticing if he needed to
Legend is that Satoshi worked on the puzzle alone.
Then later, Satoshi contacted others for input that contributed to the whitepaper (Wai and others).
After the whitepaper was released, one other individual came along to help Satoshi build the software (Sirius).
After the foundation was established around mid 2010, many others appeared to help contribute (Gavin and others).
Bitcoin Core is the most recent version of the software, starting around 2013.
That's quite interesting that there was something used before Bitcoin Core. Do we know the name of that software or have archives of it anywhere?
Also, I don't
think satoshi is contributing on the core anymore.
Satoshi never contributed to Core (that we are aware of).
1st Client: Bitcoin-Satoshi
2nd Client: Bitcoin QT
3rd Client: Bitcoin Core
Satoshi may not have contributed to QT either.
What were the release dates of these (if they are accessible)?
As this was released as open-source software, it is uncertain who has control of it and at what point due to quite a few developers working on this software in total over the time of bitcoin's lifetime.