Lots of things going on in this thread. I'll attempt to give my answers to some of the points.
1. One should have the courage of one's convictions.
This is a perfectly normal use of the apostrophe. Use of one in this way is quite formal, and possibly a little old-fashioned, in BrE. I believe that AmE would use his rather than one's, but this needs confirmation.
One used in this way has no plural, so there can be no ones or ones'.
2. Look at those apples. The one on the plate is all right, but the ones in the bowl are rotten.
We could use those instead of the ones here. I cannot think of an example in which we could use an apostrophe denoting possession with this use of the words one/ones.
3a. Nobody wants to work tonight, do they?
Nobody is singular. If the speaker were addressing a roomful of males, the question tag could be does he? However, even here, do they? would be acceptable. Once the initial question of subject/verb agreement has been settled, the speaker appears to think of the group addresed as a plural number.
3b.There is somebody at the door. They want to speak to George.
Here too, somebody is singular. The plural pronoun, once considered sub-standard, is now widely accepted as an alternative to he or she, especially if the gender is unknown, or we don't wish to mention it - perhaps the speaker in #3a is George's brother, and he does not want to let the other members of the family know that the somebody is George's girlfriend.
