Some of your question - I have no visibility into and would just be guessing. As a lender of shares, at least on occasion, the only information I have is that Fidelity is borrowing my shares (not who they are lending to), and how much Fidelity is paying me. I mentioned elsewhere - I view a lot of the short selling market as being intentionally opaque; intentional on the part of market makers and intentional on the part of short sellers.
As a market participant, I consider the opacity to be a strong negative, and a good reason to never short sell (maybe buy a Put if you want to stake out a position in that direction).
But managing the risk of the overall position is a big part of what the brokerages do, and reducing shares available for short sale sounds like an imminently practical thing to do at times. Mitigating this, is that there's a lot of money for brokerages to make lending out stock to short sellers. So they do like to lend shares too
I view the seemingly low daily shares available as more of an artificial scarcity thing. That's the number of shares Fidelity is ready to lend today at 1%. At the close of the trading day, Fidelity will take stock and make a decision on how many shares they are ready to lend tomorrow. Even if they actually have 5M shares, I don't think I've ever seen them have >1M shares available in
@vgrinshpun's reports, and mostly more like 500k.
Maybe it's a mechanism they use to constrain the daily growth in the aggregate short position - so it doesn't get bigger, faster, than their daily manual update and decision making process can react to and manage. H'mm... (the idea that the market is managed as a daily manual update is my own idea and sense of things - not something I've seen anybody write down anywhere)