improvising is how we get OL way down field when the QB passes because they are honestly expecting him to run. If there is no designed play, the other 10 guys just have to try their best and hope.
Executing what ND ran requires reading coverages and hitting a lot of shorter routes, with a lot of timing and throwing into windows instead of to an 'open' WR. There have been a lot of things called where these routes and spots were very open, so the OC has been trying to get those concepts into play. Milroe hasn't yet demonstrated the ability to execute these plays. They've been there, he's just not pulling the trigger for whatever reason.
His worst INT against UTw was a one receiver route on each side with one possible read. he threw the ball way too late, and never seemed to look at the player he should have been reading.
Reading half the field is essentially what the RPO (those are also designed runs) concepts are that Milroe was running. We've already tried that. UTw decided to play it so that Milroe would read keep instead of RB handoff, then when we'd hope that we would be forcing the D to play either the Milroe run or pass and they pretty much just played the run and it worked.
Those quick throws also require a lot of timing and accuracy and, as stated, that's a skill we have yet to see Milroe demonstrate; and he's had the chances.