It seems you can re-do them but you will not earn more points. See below image of one my modules
What I can say to you is that, if you try to complete one of them and you fail, Trailhead give you the option to try again with a penalty on points. It means, instead of getting 100%, next time you would get 75% for instance and so on.
They don't let you do that because if you could, you could just not delete whatever you built for a challenge and just keep retaking the challenge and getting massive amounts of points. Same thing with the questions. I don't know if you can combine accounts, but the only benefit would be regaining the points that you got penalized on.
Alternate solution which we all know is, you have to create a new Salesforce account and try re-doing all the modules.
Regards,
Mahesh
What I can say to you is that, if you try to complete one of them and you fail, Trailhead give you the option to try again with a penalty on points. It means, instead of getting 100%, next time you would get 75% for instance and so on.
Hope this helps.