Bioware has confirmed multiple times that it's possible to get the best ending without playing multiplayer - you just need to do the sidequests which the reviewer admitted they did not do.
That is true. Though from my reading of the PCGamer review and then discussion about how his not playing multiplayer hindered the "best" ending it seemed that it wasn't so much that he didn't do all the side quests but that he didn't scan all the planets. I think he just felt that the quests he didn't do were not important enough in terms of seeming like they would be necessary, or something like that.
Instead of farming elements for fabrication, ME3 has you scan planets for bits and pieces. An artefact, a downed ship, someone's laundry they left on their clothes line when fleeing the reapers. Then you shuffle back to the Citadel to deliver your 'finds'.
The Citadel changes at different intervals based on how many story missions you've completed. You never know if that dirty shoe you fished up off planet X is ready for a happy reunion with its owner of if you just have to wait till god knows when to once more trawl through the whole fucking Citadel hoping that THIS time some random npc will sprout an interactive dongle for you to grab onto and clear that shit from your quest log.
Reuniting these random citadel residents with their odds and ends somehow leads to tangible war effort assets (your readiness score). As does siding with particular argumentative npcs, and giving unsolicited advice or banal platitudes. Shepard's interactions on the citadel with the random residents is downright creepy.
You basically stand creepily behind someone, overhear their private moments, rush across the entire galaxy in your elite stealth ship, spend thousands of credits worth of fuel, dodge reapers, wet your fishing line, catch an old boot and then surprise gift it to someone on the Citadel.
Beating the reapers with self assigned fetch quest.... yes really. Most of those fuckers don't even ask Shepard for help.