There are plot holes though. I got the impression the assassination job Patrice did involving Severine was unconnected to Silva's main plot against M - funny, though, how she's there in the Macao casino to meet Bond. She appears to be controlled by a Tong syndicate, but but Silva also. Did Silva tell her to be there and make contact? Come to think of it, how would Silva be sure Bond would catch Patrice at the assassination site? I think he just assumed that one way or another 007 would sooner or later turn up to arrest him, as planned.
London - how did Silva know when the parliamentary enquiry would be and when M would appear, or had he, like Quantum "people everywhere?" - a good question if his plot is somehow linked to events in the film SPECTRE. Insider knowledge perhaps?
Finally the showdown at Skyfall Lodge. We know Bond set a trail of breadcrumbs - but why not send a signal to someone - Tanner, Eve, Q - to get Mallory to authorise a backup force to be on hand when, inevitably Silva and his goon squad appears (you could have them being late to the battle for dramatic purposes, but surely in real life the object would be to capture this dangerous man, not risk all on Bond senior still having a full set of guns and faithful retainer Kincaid being around - in fact it's clear Bond didn't expect him to be there.)
It's dramatically satisfying, I suppose, to have Bond, M and an unexpted ally fight off the baddie and his henchmen with little to hand, but putting the head of MI6 at risk that way is questionable, to say the least.
Since explaining "SKYFALL" is my pet project... 
- Patrice´s assassination was part of Silva´s way to make money from illegal art deals, using Severine as bait/middleman. Silva, of course, could not know that Bond would go after Patrice - but when Bond did, Silva again improvised and used the moment to his advantage. Exactly as Bond always does it: make the most of difficult circumstances, turning them around.
- The date of the enquiry has been set for some time, and as someone who demonstrates how easily he can invade any network with his computer knowledge, he got that information with no sweat.
- Why didn´t Mallory and the crew send help to "Skyfall"? Well, Dench-M explicitly says: she does not want any more lives harmed because of her, only Bond at her side. She already feels responsible for the death of the other agents and the terror that Silva has brought on everyone since his capture. She hopes that Bond will protect her and kill Silva - or, secretly, she is ready to pay the price for her mistakes.
Now, Mallory, Q, Tanner and Eve could have sent the troups anyway - but that would have triggered another enquiry with career-shattering consequences. I get the feeling from Mallory that he has no love for Dench-M or Bond, he is too much invested in politics himself, manoeuvering his career under a strict code, telling Dench-M to step down and Bond to quit in the early scenes of the film. So I do think he just does not want to involve himself or others in the messy outcome of the revenge-scenario Silva wants to live out. I even believe that Mallory is ready to risk Dench-M and Bond dying by Silva´s hand. He already is moving forward to replace Dench-M, and Bond is just another agent for him, tied to Dench-M. If Dench-M and Bond had been killed by Silva, Mallory would have been in a politically sound situation to ask his superiors for permission to go after Silva, finally cleaning up his predecessor´s mess. But as long as she is in charge, he will not interfere. Everything he can do and wants to do is allow Q and Tanner to do as Dench-M wishes (the breadcrumbs) - and this, of course, he wants to remain a secret.
As for Q, Tanner and Eve - they have no authority to send reinforcements.
