From what I can see you can max out the green bar but the EFFECTIVE readyness is HALF the value of the green bar, and afaik its MP that allows you to increase the "effectiveness" rating. Basically with single player only you can max the green bar but your effectiveness is 50%, meaning you can get 6000 on the bar, but it only counts as circa 3000(not sure what the actual max on the bar is without loading up and checking).
There are seemingly endings give or take every 1000 mark from 2000 to 6000, you can only access 2000/3000 with no multiplayer, or at least that seems to be how it worked in my game. I played, did no multiplayer had no indication AT ALL that I needed to do multiplayer, my effective readyness rating was always low but it was never explained, and I got a totally crap ending, or more specifically a totally crap choice of 3 truly crap endings.
EDIT:- gone in to check my last save, I have a military strength rating of just under 6500, a war readiness rating of 50% and an effective military strength of 3200, which is why I got the 3 god awful endings and not the other 3. From everything I've read there is no way to increase war readiness other than MP, I've, afaik missed almost nothing in terms of max war readiness rating so 6500-7000 is likely the max meaning 3250-3500 is the max you can get without playing MP at all, which means again you miss out on the "best" endings without MP.
Of course, you can, and I'm about to, use a trainer to adjust the rating before going on the final mission, not sure if it works or not. Truly epically ridiculous that a huge majority of those have finished it, got one of the 3 crap endings, been utterly let down because the game in no way anywhere maxs it clear that you HAVE to play MP to get a good ending, utter, disasterous failure on their part.
I can see it as a "anti piracy" measure potentially, HAHA we screwed all of those who can't play MP, but 99% of people buying this game are buying for the SP, and most, me included had zero intention of trying it till I'd played it through at least once as I couldn't wait to finish the SP, in which case they again failed to recognise why the vast majority of people bought and the way most of those people would play the game.
The "best" endings still sound somewhat awful and stupid, though I've not seen them to see how it pans out.