No they can't. They can keep a short position but that isn't failure to deliver.

the T+1 timer can be easily reset every day, until the market price reverts back to the Citadel's modeled price at which it is profitable/least losses for them to send order to lit market

What are the mechanics of that?

Let's say I buy a share of F on Monday, my brokerage routes it to Citadel, because PFOF.

On Tuesday, I expect to get a share of F delivered at close of business, because T + 1.

If Citadel doesn't deliver on Tuesday, what happens?

Are you suggesting they would continue to not deliver the share I purchased for several days, by saying oh yeah, we'll get toast0 his shares tomorrow? That would be pretty upsetting, and I imagine I'd call my brokerage and ask them why they're dealing with Citadel if they never deliver shares on time.

you will receive share in your name in a database, but physically it will be stored "in the street name" in the depositary house, of which there is only one.

plus even if there is only a single share authorized for stock exchange, there will be more than one in the float, due to synthetic shares: created when shares are borrowed and then reshorted, created to support derivative market (selling calls and buying puts). ALso borrow/rehypothecation mechanics is recursive, since shares are fungible, I can recursively re-borrow and re-short the same share, creating synthetic shares out of thin air, supported by nothing other than some bytes in the database somewhere, and not physical shares

https://news.ycombinator.com/item?id=26011135

The prime broker has a lot of money and will cover their customer blowing up in a net short position. They manage that with margin agreements. That isn't nothing.