Knowing which inputs come from a particular transaction and which outputs are spent within a particular transaction worsens the anonymity set. For example, if you join a round with 250 inputs, including yours, and 240 of them are owned by the same entity, and around the same outputs (value-wise or not) are later on evidently owned by one entity, then it has clearly made it easier to de-anonymize the coinjoin.
That's why Wasabi has a reasonable limit to the amount of inputs you register to a round (currently 10). Still, owning 240 out of 250 inputs in a real coinjoin is still better privacy than a fake coinjoin where you own all of them.
Everyone knows you shouldn't reuse your addresses, it's in the Bitcoin whitepaper. Whether anyone actually follows that advice or not is up to them.
Sparrow coinjoins can be tracked if postmix UTXOs are merged since you can link a single participant to multiple rounds
Why can't this happen with Wasabi again?
This doesn't happen in WabiSabi because your coins go directly into a coinjoin. In Whirlpool, there's a self spending tx0 that reveals common inputs, creates toxic change, and reveals all the rounds you participated in.
No, I wouldn't, because it is trivial to de-anonymize Coinbase outputs.
How could I know which Coinbase address you withdrew from based on which Coinbase address you deposited to? If this tracking is trivial like you claim, I would love to know how it's performed.