You're arguing against your own logic. You're saying it's stupid to blame the GK because we have conceded so many goals, highlighting our inability to cope with defensive transitions, stating how poorly coached we are (both of which I agree with) and THEN blaming Maguire. The problem was Solskjaer. Simple as that. It was all tactical.
However, I do think De Gea's impact has been massively overstated this year. He has been OK and made one or two good saves but the rest you would expect him to make. In the words of Roy Keane...'that's his job' He has already made big errors in conceding two goals against Atalanta, the Silva goal against City, at least one of the goals against Watford, and that's just off the top of my head. To not even be at Christmas and think our GK has conceded at least four bad goals is not great when shot-stopping is the only thing his proponents claim he is good at.
The statistics back up the view his impact has been hugely overstated.
His xGA vs the actual GA is basically 0, so that's neither a negative nor a positive contribution (purely based on shot-stopping). He has faced the 6th-least shots in the league, yet conceded the 2nd-most goals and his 'percentage saved' is bang in the middle of the pack in 9th.