1. I regret to say that your five years continuous residence for the purposes of ILR will only begin on your Tier 2 visa issue date of the 6th January, 2012. The test for ILR is "continuous residence". However, your period of residence under a Tier 2 visa only began when you got it. Before this, you were still subsisting on the Tier 4 visa. The visa rules allow a 28 day period of grace for a visa holder of a different visa (such as a Tier 4) to remain in the UK without becoming an overstayer. However, this 28 day grace period does not count for ILR as it merely prevents you being deported during this period. It does not count towards ILR.
2. Additionally, it is only if you are applying for a visa of the same type that any period spent in the UK without a valid visa is waived. However, even such a period (during the application) does not count for ILR. So, here, the period you spent between 25 october and 6th January will not count for ILR.
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4. I don't know where you got the date of 9th December. You can apply after 6th January. The 28 day grace period only prevents you being deported, as it deems that you are not an overstayer during this period. You were still not residing under a Tier 2 visa during this grace period. Secondly, you should apply for your dependents on the same date as they will have been five years lawfully residing in the UK on the same date as well.
5. No, you are correct. You may submit your application 28 days before the five year period is up. That is the correct course to adopt. Good luck with the application. There normally is no difficulty in getting ILR for both you and your dependents.
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