I don't have a clue whether Warburton is a good coach or not. I presumed he was a manager who had people to do the coaching for him. I don't believe for one second that any professional club doesn't do some form of defensive coaching. Saying that, you cant just suddenly rectify defensive errors with a session on the training pitch, it would be naïve to think so. You cant coach for slips, being blocked off a runner, a pull of the shirt, a poor pass or even the movement of an opposition player!
You rely on player initiative and intelligence as well as communication to organise a defensive shape, which actually starts from your most forward player.
Coaching players beyond a certain age (i found 19/20 a bit of a cut off) is very difficult. Its all relative to the standard you play at. Saying Scottish defences are crap, as stated above could be turned around to say Scottish teams attackers are brilliant.
What, in my opinion has been Warbles downfall is his tinkering with selection and formations, to the point of an experiment. That's for pre season ffs. Obviously we have had lots of injuries but we had strikers available when he decided to play with no strikers, which was nothing short of ludicrous. If he stays or goes is of no matter to me at present as i believe the ambition has to come from the top. I agree with Hammersmith when he says we need a younger manager, someone more in tune with the modern game as it is now. Who? I don't know but if we go for a footballing dinosaur then I fear we could actually undo some of the good work that Warbles has achieved.
Your point about tinkering being for pre-season still has me perplexed. I could understand tinkering towards the end of a season under two circumstances: (a) when you're a country mile away at the top of the table, have cantered to the title and have a few games remaining where you can afford to piss about a bit, or (b) when you're in the last chance saloon, desperately fighting off relegation, are losing left, right and centre, and nothing is off the table.
So, why the fudge start doing in at the final furlong, with a shot at the play-offs winking cheekily across the room at you like a young Claudia Cardinale? Either Warbles received a blow to the head, or (as I've said before) it was deliberately scuppered on the grounds that the Board determined the club wasn't ready for promotion, both financially and in terms of playing staff. The latter sounds ludicrous to me, but stranger things have happened.