Well the version I have doesn't allow the changing of difficulty settings (never seen one which has actually, anyone care to elaborate?) but I know exactly what you're talking about.
I've found that those Sweepers actually respawn, I killed them once and by the time I returned from the Airport puzzle they were there again so don't bother trying to kill them, a good tactic I've found is to stay still a second or two and wait for the one on the right (I think, pretty sure it is that one, it stands closer to the door) to start jumping. Once this happens make a forward dash for the double doors and you should be okay hopefully, they'll lunge at you but will literally fly overhead. As mentioned above a zig zag generally works best for most encounters.
As for the ammo situation, Zombies are probably the easiest to avoid/get through in CV/CVX for a few reasons:
- The knife has an amazing multi hit abilitiy, always aim downwards
- If you mash the buttons fast enough you'll escape unharmed (you will get bit eventually doing this though, I think it's every 3 grabs they'll get an automatic bite).
-If you save all of your Handgun ammo in the first parts, avoiding every enemy you can (which is pretty easy) then take the Lockpick and obtain the Custom Handgun parts and combine it with Claire's Handgun. it can really decimate even Hunters and Sweepers, I'm not kidding. I was rather suprised at how fast it killed them with burst fire on.
The reason I say Zombies specifically is because there's a hell of a lot of them in the game, if you ignore the majority (or all) of them then you save a lot more ammo than you'd think. My point is basically that you should only attack what you really need to attack, on my last run I had a stupendous amount of leftover ammo by the end, (80% Assault Rifle, 33 Shotgun Shells, 6 Acid Rounds, 6 Flame Rounds, 17 Grenade Rounds, 268 Handgun Bullets, 15 Explosive Bowgun rounds) I didn't even bother to pick up the Magnum or the Machine Guns because quite frankly they're rather superfluous at that stage in the game.
Sorry for the text wall but I hope this helps you somehow at least.