Slow line from Slade Lane Jn into Picc 10 or 11, SL back out
That involves crossing the Up Styal and Down Slow at Slade Lane Jn, and then the Up Styal again (at that point called the Up Slow) at Ardwick Jn or the Picc throat.
In reality you'd do as the Cleethorpes-Manchesters do now, and go on the Down Fast until Picc, where platforms 9-12 are conveniently in the vee between the Fast and Slow lines.
So it's conflict-free when going to the Airport, but there is still a conflict on the way back (you need to cross the Up Styal/Slow somewhere between Slade Lane and the station throat).
Can only be done with a single 185 and imports significant performance risk - if the train is even 2 minutes late it's going to delay the surrounding trains.
how is that more challenging than a terminating service spending 20-40 mins in a platform and changing crews?
It involves a needless conflict and has very little resilience for delays. There are few crew changes at Picc for these services with the Cleethorpes service now terminating there, as the trains are entirely Sheffield/Cleethorpes crewed and mostly involve out and back trips to Manchester on the same unit(s).
£100m on the Ordsall Chord for one tph?
Poor value in hindsight, but the scheme also involved other upgrades (such as more extensive bidirectional signalling around the Water Street Junction area). Either way there's no point falling to the sunk cost fallacy.
Splitting services at Warrington Central - it's rare for delays to be caused by late running locals, it's the EM from Norwich, or the Airport express that cause most delay - and late running Nottingham or Cleethorpes trains would still disrupt the split service (shunt to WAC sidings, wait for late express to go through). The Glazebrook down loop provides a fairly efficient way to mitigate delays westbound; why hasn't it been possible to have an up loop at WAC? With only one tph local calling all stations WAC-MCO, what happens when one is cancelled?
The CLC is going to remain a thorn in the Corridor's side until it's sorted out properly once and for all - i.e. electrify the line, put 323/331s on it, and give Warrington Central a central turnback platform, preferably two bays facing east and west.
However, to put it mildly, that's still a long way off.
EDIT: I dont understand also how platforms 15/16 at Manchester Piccadilly is going to do much use when we cant add tracks between MCO and Piccadilly, can there be any other solutions, such as using disused lines or building new lines?
The minimum dwell at Piccadilly is 2 minutes. At Oxford Road it's 1 minute off-peak and 1.5 at peak times. Add on the 2 minute headway and you can see that the maximum theoretical capacity of the corridor is 15tph (323s have a derogation to stop at Picc for 1 minute but it's not ideal).
If you have 2 platforms in each direction at Picc and Oxford Road, with sufficient overlaps, your theoretical capacity doubles to 30tph, as the constraint is now only the headway rather than platform availability. Of course trains stopping at Deansgate, trains coming out from platform 5 at Oxford Road, as well as the many flat junctions and the length of freight trains, will limit this in practice, but at least the constraint would no longer be how many trains you can fit through Picc & Oxford Road.
Unfortunately the longer that the DfT prevaricates, the more expensive and time consuming it will become to build extra platforms, as more and more skyscrapers are built in the way.