A defender opt for a tackle usually because he knows he wouldn't make it if he run. Even with the tackle Harry couldn't make it I don't think he could step into the path of Almiron's run. You could notice Almiron was not runing in a straight line, he was running away from Harry and Harry, knowing himself couldn't make it opted for the tackle. But he was unsuccessful. That's all I think.
If you watch the whole situation again you'd see it's really hard to blame anyone for that goal. Our whole defense and midfield was caught off because of a bad clearance that somehow ended up with Almiron. Imo you can say there's some lack of concentration with Matic and Shaw though. They should have keep a closer eye on Almiron.
It's hard to judge because we don't have data on speed, direction or whatever. We are trying to judge it by eye. I think Maguire can make a foul there. Almiron jumps over his tackle, if he went in with his body rather than his legs I think he can body check him.
But if you are right that he couldn't, because he's too slow, then he needs to drop off.
You point out correctly that our defence was all over the place when that move started. Shaw was closer to the right touchline than the left, Matic was ambling about, Varane was covering half the pitch. Harry makes the call to go in rather than start running backwards. If he knows he can't win the ball he has to try to get back to help Varane, otherwise Varane is in a 2v1.
You are not wrong to point out that it's not as simple as saying Maguire made the mistake, that's why Newcastle scored. But his decision was probably the key one given how out of shape we were. If he grabs Almiron's shirt, even after Almiron's jumped his challenge I don't think he gets a red. Too many United shirts about. But it stops the counter.
When I see City defend counters, a lot of the time they don't even try to win the ball. They just stop the man, give away a free kick and reorganize. We need that same kind of streetwise approach.