Tier 2 general extension- Query regarding RLMT

[scher]

Hi,
I did come to uk on 13/09/2009 for my masters,have finished it, got psw visa from apr2012 to apr2014.I got Permanent job as a 'web based software developer' for nhs and I have applied for tier2 general visa on Jan2014 (with soc code 2137 ,my salary was 21,300 )through premium service appointment( I have paid 970 pounds approximately).Then I got my Tier2general visa from 24/01/2014 TO 14/04/ 2017. I also applied tier 2 general dependent visa for my wife from India as I got married in aug2014 in India and she got the visa from Nov 2014 to 14/04/ 2017..
My current salary is 25,500 and I am in the same job with the same employer..In order to extend my visa for next 3 years as my visa is going to expire onn 14/4/2017, I have requested my employer to sponser me,and then my HR applied for unrestrcited COS with wrong soc code 2136 and they have forwarded me that COS number.
I have realised that they have applied to unrestricted cos with wrong soc code, hence I have requested my HR to cancel the COS and then apply for the new COS. Then again HR applied to unrestricted COS with the information as mentioned below
SOC CODE: 2137,
job title as WEB BASED SOFTWARE DEVELOPER.
Current salary: £25,500 p/a ( 37.5 perweek)
Job start date: 15/4/2017
Job End date: 15/4/2020
IS the current job under shortage occupication list: No
Is Resident Labour Market Test done? : No
HR applied to new cos with the information on 22/12/2016 as above and they have send me the new COS number on 22/12/2016 it self ie same day.
Now I am going to apply for tier 2 general extension along with my wife. I would like to know that
1)when should I start my application process ie how early i can apply to my visa extension
2) Do I need to go through Resident Labour Market Test ???
3) My actual job start date was 18/12/2013 but HR mentioned me that she could not enter the date in the past hence she mentioned my start date as 15/4/2017 as my current visa is valid until 14/4/2017....is this is going to be a problem???
Please suggest me on above three queries...

Thanks and Regards,
Scher