Depending on how the stats are compiled, the risk of being in an undiagnosed high-risk group is included in the risk for the general population.
If you later are in a situation where you will have greater risk, you can often get the vaccine when that situation arises, and probably won't be any worse off for having waited. Vaccines become less effective as time goes on, so you might have to renew the vaccine when that situation arises anyway.
I am unsure how immune disorders factor into this.
For meningococcal disease, the most volatile risk factor is sleeping in rubbish communal bedding (or living in tropical Africa, or both). In this case, the risk goes from a tenth of a micromort per year up to maybe one micromort per year. Some situations where that might happen may be out of your control, sure. The only plausible non-society-crushing situation I can think of where that might be problematic (i.e. access to the vaccine would decrease) would be if you were to become homeless for lack of money. Feel free to adjust accordingly.
The travel risk is just as you say.
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Don't be shy, everyone has holes in their knowledge, though the fewer and the smaller we can make them, the better.
Please be respectful of other people's admitting ignorance and don't mock them for it, as they're doing a noble thing.
To any future monthly posters of SQ threads, please remember to add the "stupid_questions" tag.