I'm seriously getting peeved at how my Seviper chains keep breaking. In the process, I think I discovered another rule:
If there's more than one patch which satisfies all the other rules, reset the radar.
I also believe that a patch is safe even if it's only three cells away (as in, outside a 7x7 box with your character in the middle). It's also safe to enter a patch at a side/corner, and such patches should be considered "safe" for my above rule. (That is, if there are two viable patches, both of which are far enough away, and one is on a side/corner, reset the radar.) The only problem with patches at sides/corners is that you're likely going to have to reset the radar after going into them, because you'll usually get less than four shakes.
Also, considering how a chain can be broken even by the Pokémon you're chaining, I get the feeling that Static/Magnet Pull don't help past the initial encounter that gets the chain going. So it's probably safe to switch in a Synchronizer/Cute Charmer after that point.
Ugg, still chaining Sevipers. My longest chain is still 16, which doesn't even register in my Poketch.