1) Mixer knows BTC address from which you send and knows the address to which you receive. Is information about the connection of the sending address with the receiving address saved? If this info is stored on their server, having access to this info, you can unravel the entire chain of transactions?
It'll be impossible to verify whether this data is stored by a mixer or not. Most say they won't keep logs or will only keep them for a certain amount of time (chipmixer is a week unless you destroy your session and then it's straight away - from what they say).
2) If the mixing service provider is a website, then it can get various information, such as an IP address and location? What can the mixer learn about its customers in this way?
They can get your ip and location. It depends on what your isp stored about you but I've seen cases where you can get someone's name/address too via whois sites.
If you use tor or use a public WiFi service then you'll be much more hidden.
3) What information about users can generally be stored on bitcoin mixing service servers? If attackers / hackers get access to them, then this can be used to the detriment of users, right?
Most mixers save session tokens and deposit addresses, these are things they have to save to allow them to support their users - aside from that and maybe a captcha solve, they shouldn't be storing anything else. Captcha solves might hide some information about who a user is but it's probably not reliable.
4) How can minimize the collection of personal information by mixers?
As said above, use tor and/or public WiFi where you can.
Don't take your funds straight out of a mixer as it'll be easier to use chainanalysis to determine it's you.
If you wait anywhere from 2-7 days before withdrawing funds from a mixer they should be harder to trace.
Also if you use multiple mixers or multiple services, it should make your funds harder to tract too if worried about logging - it may be possible to incorporate dexes too.