Again, I don"t know for sure, but my take is as follows.
You can play in any of the regional events - but obviously only at one on any given day. I don"t remember reading anything that says you can"t play in any of the regions in subsequent events. Might be wrong though.
As a satellite, you get in the top 10%, you get a seat in a national event. You also get points - and these go towards your season total (your best 20 results count, or something like that, so you can miss events and not worry too much). So even if a venue only attracts 30 runners, it"s going to be just as difficult to get a seat, as you need to finish in the top 10%, but it would be easier to score points (unless they too are allocated pro rata of a full field of 80 runners).