What is the process for retroactively (post completion) reviewing plagiarism on a dissertation?
The scenario of an investigation for suspected plagiarism in a PhD thesis is so rare and unusual that I'm pretty sure there is no "typical" recipe for how it would unfold. However, I'm familiar with one such case that happened some years ago at my institution, and I'll take that case as my definition of what "typically" happens.


  (1) Would such a review process be limited to the dissertation itself or would all the work of the student be reviewed?


It might, if the student's other work was available to be inspected (which I think is doubtful since the student will have already graduated), and if there was some concrete basis for suspecting plagiarism in that work, and (which IMO is the least likely condition to be satisfied) if the people involved in the investiation had enough time on their hands and were energetic enough to pursue such secondary investigation leads.


  (2) Would such a review involve questioning the academic advisor/research supervisor? What about the thesis committee?


This would be highly embarrassing to the people you are mentioning, so I expect they would only be questioned if there was a strong basis for believing that the information they could provide would materially affect the outcome of the investigation. Thus, the advisor might be questioned to get some background information on what took place when the student was writing the dissertation; the thesis committee members, almost certainly not.


  (2a) On a sidenote: Would the thesis committee for a doctorate degree be obligated to check the research in detail or are they allowed to rely on the research supervisor for that task?


See the comment by BrianDHall. The thesis committee never "checks the research in detail", and in all except a rare handful of cases, neither does the research supervisor.


  (3) Would such a review look at the theses of other PhD graduates who were under the same supervisor?


This question suggests to me that you have watched one too many episodes of late-night TV crime shows. Unlike such shows, universities aren't populated by dogged investigators (with checkered pasts and colorful personalities, usually divorced and heavy drinkers) who work day and night, dedicating their lives to exposing all instances of wrongdoing and then bringing the perpetrators to justice.

In other words, the answer is no.


  (4) What happens if such a review finds a doctorate is academically or ethically flawed? What, if any, penalties can be invoked against the student?


Given that the student has already graduated and is no longer a member of the campus community, the only punishment the university can actually impose is to revoke the degree. The only real question to debate would be whether the student would be allowed to come back and rewrite the dissertation if they wished to. Moreover, given the severity of the offense, and the risk posed to the university's reputation and credibility by PhD degrees it grants that are based on plagiarism, revoking the degree seems like a completely appopriate and logical response.


  (5) If the outcome is to void the degree would the student be offered a chance to comment/defend or would the degree be revoked without warning?


Undoubtedly the student would be given a chance to defend themselves against such an accusation before any action such as revoking the degree is taken. They would likely be entitled to ask for the case to be reviewed by a committee, and to have a panel convened where they could present evidence in their defense. Given the high stakes involved, if they wished to contest the accusation they would likely hire an attorney.


  (6) How long would such a review typically take to complete?


Probably a few months, but I assume it can vary a lot and take longer in some cases or be completed quite quickly in other cases.