I'd suggest only 3 times but I've seen it split up 4 times. personally I feel like it's more of a hassle and a pain when trying to siphon out the tank.
As for stressed middle fish, after about a week or two they all get used to each other and realize that they can't get at each other and will eventually not care about their neighbors so it's really not too much of a stressor for most fish although occasionally you'll get one that's spazztastic and will freak out at any other male and may bite his own fins in 'retaliation' so you'll just have to watch out for that is all.
For a split 10, I'd suggest just a regular hang on back (HOB) filter system with a water bottle baffle like so: http://www.bettafish.com/showthread.php?t=30139
and here's what mine looks like on my split 3 way 10 gallon: http://s1292.photobucket.com/user/li...ilter%20Baffle
that way it spreads the water out to the sides, creating a pretty even flow through the tank so it spreads the heat as well. You can do that in your middle section to make it work better but I just have mine at the end with another small internal filter on the otherside facing in to create more flow in the water
And do you mean filter media? Substrate is like gravel, sand, dirt etc. Filter media are things like the cartridge, foam, carbon stuff like that that goes in the filter. And usually you don't need to change them as often as companies say you do (which is a month). I have one cartridge I've been running for about 5 months now and it's still going strong! The secret is the swish the media out when you do your weekly water change. You take out the media and swish it around/rub it in the old tank water you just took out so that it get's all the dirt/mulm/detritus out of the cartridge and it doesn't clog up as fast. And when you replace a cartridge, preferablly stuff another cartridge in there about two weeks before you take out the old one so that your Beneficial Bacteria can transfer to the new cartridge.
That's another thing, you should go to the Betta Care or the Betta Habitat section and look for the Nitrogen Cycle sticky and learn about Cycling a tank. And not the "24 hour" cycle period the stores suggest, that doesn't nothing for your tank except tells you your filter and heater are running fine >.<