You can switch invasion mode off right?
I mean it sounds great but some 12 year old dick riding round in a kubelwagen trying to knock you over is not my idea of down time
Yes, you can turn it off.
I read a bit more about 5 last night. They added the ability to play entire missions co-op which is kinda neat but they stripped out the co-op spotter/saboteur mode from 4. That's a shame because it was really fun. One player acted as a sniper/spotter and had to spit and tag objectives for the other player, the saboteur. They would have to sneak around and try to complete said objectives. It player out a lot like a FPS version of Commandos 2 but there were only a handful of maps featuring this mode.
The other thing they removed is the tripwire mine. Speculation is that it was too easy to take out other human players in invasion mode with these so they got rid of them. Big mistake IMO. They should definitely be in-game if you turn off Invasion mode as they're invaluable tools for covering your flanks in single player (and it's fun to blow Hitler's nether regions off with them on the kill the Fuhrer maps.