Why couldn't Bond have backup at the battle of Skyfall?

From what I can remember:

• When Bond "kidnaps" M she says:

  Too many people are dying because of me.

  Bond suggests that they use Silva's desire to kill M to their advantage. M agrees to play the bait, but insists:

  Just us, no one else.

  Bond then asks Q to create a

  ... trail of breadcrumbs impossible to follow except for Silva.

  When Q asks if this is official, Bond answers:

  Not even remotely.

  So, Bond and M going rogue explains why there are no reinforcements.
  
  

• What Bond didn't know was that Skyfall had been sold when he was believed to have died.

  When Bond asks Kincade about the gun room he learns that all the guns had been sold to a collector weeks ago (except for Bond's father's hunting rifle).

  This explains why they had to improvise.

He wanted Q to make up a trail so that only Silva can follow them and not the whole MI6, who were of course also in search for M.

Now Bond wanted to leave the rest of the government out of this, because they would make this whole thing a large bureaucracy act and in the end wouldn't allow M to be used directly as bait. They would rather secure her and investigate this whole situation. This in turn would lead Silva to hide again, resuming his evil doings (and maybe even publishing more agent names), until he one day comes up with another ingenious plan to kill M.

Bond just wanted to end it. So he needed to act fast and the situation to look reasonably easy for Silva, so that he confronts him in a direct fight, without going into recluse again and making evil plans.

As stated in the film Silva was always one step ahead of them. Bond implies that there was a mole at MI6, and if M put her safety in the hands of MI6 she would surely be killed by Silva.

The only person Bond trusted at MI6 was Q, but he knew that no matter where he took M that Silva would quickly follow. So he asked Q to create a fake digital trail to take Silva on a wild goose chase, and this gave Bond time to escape to SkyFall.

Bond took M to SkyFall because he had no where else to go, and it was the last place anyone would look for them. While it might not make sense to hide from Silva at SkyFall. It was a good place for Bond to hide from MI6.

I'm not sure Q's job was to get Silva to SkyFall. I don't think Bond ever told him where they were going, and to ensure they were not tracked digitally they switched cars.

When Q was entering the data in the computer for Silvia to follow. There were three people (including Q) in the room at the time. One of them must have tipped Silvia off to Bond's location.

EDIT: Thugs show up at SkyFall without Silvia. It's possible they were there to search for Bond, and when they started fighting one of the might have contacted Silvia. This explains why he shows up later arriving via helicopter.

The reasons for this are quite simple:

1. Silva's vendetta is personally against M, who he felt betrayed her. Bond is M's loyal protector (the British bulldog shown by the figure), who by this point has enough personally against Silva to be part of the party. M doesn't want anyone else involved as enough people have died (remember the flag strewn coffins) due to HER actions

2. Q isn't aware of where Bond/M are going, he is only asked to buy them time by laying a false trail (which is designed to be too complicated for anyone else to work out to again protect the innocent). The implication is that given how far ahead Silva has been so far he WILL work out what where they have gone, this is purely a stalling exercise (so they don't get set upon in the mid journey, possibly in a built up area)

3. Bond didn't have any confidence in his colleagues (remember he'd just been "dead" for a while after being shot by one on his last operation)

4. The choice of Skyfall was to simple, Bond had spent his childhood there, and knew the place (and all its secrets) intimately. It is the one place he could be sure to have an advantage over Silva

5. Bond was also experienced enough to know that for all his henchmen, this would become one-on-one between Silva and him, might as well have the home advantage.