The route from Falmer Station to Lewes is alongside the A27 trunk road. Its a 70 mph unlit dual carriageway.
It does have a combined footpath/cycleway but is only about a metre wide in places. It would not be a safe route for hundreds/ thousands of people after dark.
I don't think there would be hundreds of thousands of people trying to walk the 5 miles between Falmer and Lewes. 95% of the population won't walk more than a km or so. you see this at many country scenic spots... crowds of people within a few hundred metres of the car park, walk for 15 mins and you're by yourself. There would be a relatively small number of people walking the distance from any of those stations surrounding Lewes.
The last time I went to Lewes bonfire night we walked back to Brighton afterwards. The first bit along that road, then over the downs. There were a few other people walking along the road bit, but not crowds, just a scattering.
I've no problem with the principle of making it a bit more difficult for people to get to Lewes. But why apply this disproportionately to people who use public transport? Anyone with a car can park up a bit outside town and walk a relatively short distance. If the next-nearest-to-Lewes stations were open, it would mean that those without cars would have some opportunity, if dedicated enough, to walk a few miles and still have access. Still much less easy access than car owners but at least some sort of chance.
I really don't see why there should be a worry about massive crowds walking from somewhere like Falmer. If that number of people were willing to walk, then surely the same worry should apply about people parking up in cars several km away from the town.
Sure I can see that closing those stations is an easy way for the police to remove a (probably over-stated) worry. I imagine it's done because it's easy - fewer people will make a fuss than you'd see if a wider area of roads were shut down. But I'm glad to see it questioned here.