The midfield is still not up to scratch after him presiding over 3 transfer windows, and 18 total months in-charge. You compare our midfield with some of the elite ones across Europe, and it is still lacking in quality and sustainability over 3-4 seasons despite spending about £60 million on 3 players. Carrick will be 35 at the start of next season, and Fußballgott 32. That doesn't bode well for the medium term future, and meanwhile Ander sits on the bench. This is not what you'd ideally want in a midfield overhaul when Dortmund for example have spent about £12 million combined on a United 'reject', and Julian Weigl to reshape theirs. Too much money has been spent relative to the output, which has been a consistent theme under Van Gaal. Even aside from quality, the tactic with 2 sitting midfielders in front of the defense seems excessive from a defensive standpoint; and not penetrative at all, especially against rubbish teams that park the proverbial bus - something that is reflected in the results where even aside from a lack of goal threat, our midfielders don't pose a substantial goal threat.
As to your second point:
Chicharito is still good enough, but doesn't fit into the manager's 'philosophy'. Perhaps if he were more accommodating and willing to bend his ideology to suit the personnel instead of shoving his vision down our throats, we could've gotten something out of him - worth remembering that he has scored 12 in 17 games for Leverkusen, and the Bundesliga is a Top 3 European league. Fun fact: He has scored 1 fewer goal in Europe than Luis Suárez and Robert Lewandowski, and as many goals as Rooney, Martial, Mata and Memphis combined. Even if he doesn't fit into your possession based system, Hernández is a top notch goal scorer and would've been an excellent Plan B - infinitely better than the Fellaini charade.
Nani would've done well but he's too inconsistent for the manager, and would've been dropped repeatedly because he tries and sometimes he fails; instead of our current approach where we don't even try to begin with, and recede into our shells. Van Persie is still better than Rooney IMO. Januzaj would've added a fresh element to our attack, even as a substitute. So there were players who could've brought something to the United team but we let them walk instead. That is squarely on the manager's shoulders because at United, he is the ultimate authority when it comes to personnel decisions.
To the last point:
The team can't always have top players. You don't need elite players to play attractive, attacking, free-flowing football anyway. That's where managerial ability, game-planning and adaptability comes into focus. Van Gaal has top shelf managerial ability no doubt given his record, but the second and third part is where he fails time and again, historically speaking. Too many orthodox schemes, being stubborn to a fault, not altering your approach to suit the cards you're dealt. And that is what is blighting him at United too. The team can play much better football, you don't need a Suárez type #9, and Neymar/ Costa on the wings to do that.