Mass-effect – Is it possible to get the best ME3 ending without Live Gold membership

I'm playing with XBox360, no XBox Live Gold membership. 2nd-hand game, but there was free download of EA online pass, so I just redeemed that (but of course can't play multiplayer without Gold), and have the account successfully linked at N7 HQ.

In the game, I am currently at the point just before the end game phase starting with the assault on Cerberus space station and the Illusive Man, with total military strength ~5600, and the default readiness 50%.

While carefully avoiding spoilers, I've managed to read that I need effective military strength 5000 to get the best ending, so I'm like half the way there, and not many uncompleted quests.

Questions, considering current date and details above, in order of importance:

• Is the 5000 point requirement true? (And yes, I realize it doesn't buy me much, but I consider it a bit like missing last page of a book I've been reading since ME1 came out…)

• Is it possible to reach the best ending in single player only? Briefly, how? Like, is there some part of the game I can grind to get it?

• Based on my own research, it seems I could buy Mass Effect: Infiltrator for Android, and get galactic readiness up by that. Now the question is, will it work in practice? That is, can I really get enough military strength and readiness rating, considering where I am now? Can I do it without in-app purchases on top of the game price itself?

• If I get a Live Gold pass, can I actually play the online multi-player still, find other players available for play with a newbie, when it's been so long since the game was released? How much online play time would it require to get to my goal?