some questions regarding research plan for postdoc
You can cover several years with one research plan by putting the different projects on a fictive timeline.

You can mention multiple of them but they have to have a logical connection.
I don't have experience in mathematics (I did a PhD in Life Sciences), but one important point for postdoctoral application is to show that you can do research in an independent manner i.e. that you understand what topic of research is interesting/relevant, the hypothesis you develop regarding this topic, the approach you choose, and the key experiments which would lead to the results to answer your hypothesis.

An another point is that the research can be done during the timeline of the granted money, i.e. it is realistic and achievable. It is also hard at this stage because you might not know yet but you need to have an educated guess at it. That's why that in terms of number it is also up to you to decide if you should include all of them or not.
Regarding possible failure of the plans, try to include the ideas which are innovative and challenging enough but at the same time easy for you (because of your PhD experience) to do in realistic timeframe.

In your case, the hardest part of the grant proposal is to convince the reader that there is a logical approach between these different parts you want to study. If there are no connection at all then it will be difficult to do so and I would advise to not include all of them.