Theoretically in IRT, $\theta$ and $b$ (which are measured in the same scale) do not have lower or upper bounds. They can go from $-\infty$ to $\infty$. Sometimes a test is unable to capture the full extent of an examinee's knowledge because it doesn't have enough items in the lower or higher ends of the difficulty scale, for example. Anyway, it is not a straightforward task to convert from $\theta$ values to percentages.
One thing you can do (and I know of a national test in my country that does something similar) is to transform your data using a desired mean and standard deviation. For example, you can displace your mean from 0 to 500 by adding 500 to all examinees $\theta$ values and changing your scale accordingly. You can then present their scores as if they went from 0 to 1000 (which you know that theoretically is not the truth, it just means your scale is centered at 500).
Ultimately, you should have some kind of interpretation of your difficulty scale, by analyzing the items in the item bank and trying to separate them into different categories (which is what you ask in your second question). Then, you can use these categories to report your students progress. This analysis can be done by specialists in the topic of your test. They may use anchor items, for example (items which are very representative of what contents students should know) and not be fully guided by statistics.
After you have these categories, students who are at the lowest level or below it might be considered as knowing 0% of what you intended them to know, and students who are at the highest possible level the test can measure may be considered to know 100% of what you intended to know at that point in their education. The other students can be placed in between.