On the High Veldt at Johannesburg And Centurion I would go with
Abbas batting at 11
Shaheen Shah batting at 10 (for the pacy, bouncy break-your-Hand Mitchell Johnson effect)
Amir at 9.
Faheem at 8.
Shadab at 7.
At Cape Town, where spin can have an influence, Id introduce Yasir Shah for Shadab if Shadab was ineffective at altitude in the first two Tests.
Triple A means Abbas, Amir & Afridi - so, at least for once we have agreed on something regarding team combination.
I don't disagree on your thought process, problem is you often over estimate the capability of young PAK players - Boss, this in not 80s/90s - as a boy, I also used to get surprised, how they can pluck someone from thin air almost every series like a magic and that newcomer looking like million dollar baby. These days, PAK youngsters scores lot more in PakPassion rather than in cricket field - and these days, they call 35 years old middle aged man "youngsta buti". These Nawaz, Shadab, Fahim ... are not Tauseef, Mushtaq or Abdur Razzak .... let alone Wasim, Waquar, Qadir, Saqlin, Inzi or Saeed - control your emotion.
Shadab came with outstanding potential - but went exactly the route prohibited for him (No, I am not telling about T20 only). Didn't develop on leg-spin, neither batting enough to make it as specialist bat - now he is heading towards V2.0 of .... no not Afridi; Lala of 1998-2013, would be the MVP of this team in 3 formats. I don't know, if you are aware of Mansoor Amjad - guy was turning miles in 2004 U19 WC & I saw him from VIP stand at Old Dhaka stadium, then won couple of games with bat as well - now he is probably trying to make some Regional team at the age when he should have been at his prime. I am sure, our memory of Usman Qadir is it still fresh........
Had Shadab been really tight, I still would have given him a BoD on potential. But, he simply has lost the bowling plot - will go for plenty, and in low scoring Tests, you simply can't buy couple of wickets. You don't pick such spinner whom you can't back to give control on Run Rate - in that case bigger damage will be that Abbas or Amir will be bowling to contain scoring rate. In his last 2 Tests against ENG (for once, I hope at least you won't bring IRL Test here); in 2nd innings PAK bowled 82 & 106 overs - his figures were 19-2-63-2 & 17-2-50-1 (add 6-0-34-0 in 1st innings at Lord's), this is simply not what you can get away with from your leading spinner in AUS/SAF. These are not UAE that 3.25 RR means batsmen are racing - one bad session can leak 150, and the Test ends there. Moreover, since ENG tour, my feeling is, he has declined considerably.
It's romantic to think a Leggi bowling on foot marks with 350 to defend in 4th innings - but it won't happen. PAK won't post enough for any spinner to come into play on 4th innings - in fact, I won't suggest to bat first if wickets are like IND series - Amir might come into bat before lunch. It has to be strike by pacers and to keep them fresh ( & manage over rate), you'll need one spinner or a very tight medium pacer to bowl accurate 35 overs at least. At present, I don't see anyone but Yasir as spinner, therefore I won't pick any spinner for 1st & 3rd Test. That tight darting job Haris can do well.
Another theory I have to reject - bottom half very rare cases can recover when new ball is still moving & top 2 pacers are tiered - for that, you need to reach at least 35 minutes after lunch with 5, max 6 down, and one set top order has to be there as well. The batting line-up you are suggesting ends with Sarfraz at 6, then Shadab, Fahim, Amir, Afridi, Abbas ....... Last time Styen & co got PAK 7 down before lunch @ J'burg, it ended at 49 - that too on 2nd morning. Had SAF enforced follow-on, Test won't have seen 3rd morning.
If I am to take a chance, I'll take it with Abbas's durability, and Haris keeping things tight at one end.
Imam
Azhar
Haris
Asad
Babar
FZ
Sarfraz
Hasan
Amir
Afridi
Abbas
This was my XI for 1st & 3rd Test, with Haris, Azhar & FZ bowling few overs. But, may be Fahim can replace Hasan as the work horse.
For Cape Town, drop Fahim/Hasan for Yasir. If Shadab is to play, he has to make it as batsman, which I don't think he is good enough.