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I don't use User based licences, but my interpretation from reading the EULA is
1) Providing when you refer to multiple environments, you mean a production environment, and then a 'dev/test' environment then 'yes', a named user can use the same licence on both environments. The licence is associated to the individual for use on both a prod & non prod environment, so if you've purchased 20 licences, you can't have 20 named users licensed on prod with say 5 other people using the licences on dev/test.
2) Yes you can reassign the licence to a different indivudual (eg if someone leaves your business say), but you shouldn't be doing this on such a regular basis so that you're essentially 'sharing' a licence
The EULA I've referenced can be accessed from here : Legal | Tableau Software