I know what you're getting at but I submit that because, in the event, the attack wasn't properly planned, positioned or led that Lee's confidence in his army was misplaced—the failures were in the very nature of Lee and his army.
You make three statements. Let's examine them:
Was the attack properly planned?
The plan is simple. An echelon attack falling like a series of domino's working its way up the Union life from its left flank all the way up to Cemetary Hill (Lee's real target). At the same time Ewell would launch a series of diversionary attacks on the Union right flank so that reinforcements can't be sent from there.
In theory this should work perfectly. However it is created before Midday (accounts differ as to whether it was created before or after Lee left to go to speak to Ewell - certainly it was being mentioned beforehand but as Longstreet (later) claimed there was no consensus for an attack until after Lee's return. (Even then Longstreet claims not to have been in favor though he does - slowly - set the plan in motion. As a result the troops dont get into position until 3/4 PM and by then the situation has changed markedly (Sickles). There is also the problem that the intelligence report that the plan is based on is dubious to say the least. However even with this if the plan had been adhered to I think there is a reasonable chance of success.
Was the attack properly positioned?
In hindsight which is an exact science no. But it was not egregiously wrong.
Was the attack properly led?
The problems as I see it are in the Leadership and there is plenty of blame to go around to borrow/paraphrase a phrase from the esteemed Eric Wittenburg (admittedly not one he used about this particular incident)... but still it seemed apt.
To begin with Lee seems to be having an off day. Everyone is allowed one now and then but for the ANV it will be a major problem. As a result when Longstreet and later Ewell start questioning his judgment he doesnt take it well and apparently does something that he just doesnt do. He issues orders to McLaws that bypass Longstreet. This runs counter to his usually laissez faire command style.
Longstreet appears to get in a huff about this and pedanticly follows his orders to the letter. (The Countermarch. Waiting on Laws brigade) He also does not seem to have bothered scouting either the route or the location of the attack that at the least he was aware Lee was contemplating attacking.
Ewell is being equally hard headed and pretty much refuses to give up the ground he gained on Day One. He also delays his diversionary attack until way after Longstreet's attack has begun. Thereby negating its effectiveness as a diversionary attack but increasing its own chance of success. A coincidence? I think not.
A.P. Hill. Who? Where? The mystery man of Gettysburg. Yes he is new to Corps Command but especially on Day 2 and 3 he doesnt seem to bother... May also have been ill but that mostly seems to be post war speculation. Lee was also supposedly keeping an eye on him... If so it isnt very effective.
Hood and McClaws then perform magnificently, the echelon attack that is two brigades deep along the front doing all that can be asked of it and more in that it attracts troops from 6 Union Corps! Only Eleventh and the Cavalry don't send troops. Ok its not perfect and Hood going down is a major blow but even so... 6 Corps!
Then comes Anderson's division. Having a jolly little picnic (literally) while your division is going forward is not generally considered to be the right thing to do. Having only 3 and a bit of your 5 brigades charge is also inexcusable. (The troops that do charge do so
very heroically and their sacrifice is oft overlooked) They also reach Cemetary Ridge in places but without support (Mahone should never have commanded again after his shameful refusal to advance IMHO) are persuaded to retire.
Rodes/ Early do well but it's supposed to be a diversion so havent brought enough men when they encounter unexpected success. This is quickly snuffed out.
Allegheny Johnson is unfortunate/ a victim of Ewell's decision to delay the attack in that he comes across bad terrain and faces the only fortified Union position. Then darkness comes robbing him of the ability to attack/ see quite where he is and how vulnerable the Union supply lines are. (This has changed by first light).
All through this Confederate troops fight bravely and yet the people above them have some serious questions to answer and IMHO that starts with Lee. However in the Confederate Mythos and especially the post war Lost Cause Mythos he is bulletproof therefore the problems must be elsewhere...
So I would say Lee is right to have confidence in the fighting abilities of the
men of the ANV. Maybe he is not so correct in having confidence in some of its leadership... and perhaps that day his own...
EDIT - punctuation/ clarity