Hey Anish,
So again just from what I have been advised, if you can prove via references and payslips, tax history and bank statements etc, Immigration will take the 1st 3 years of your work history as 'training' and the rest should count towards your 'experience'. DON'T make the mistake of claiming for years of experience for the whole time otherwise you will come up short on points.
I supplied them with my Gas certs back to 2002 and work history/references etc from 2005 to present so was well above the 10yrs even if they knock years off for 'training' for 15 points.
Which is annoying as I started in 1997 but the company I worked for and was apprntice at has long since wound up.
For the EOI I just put the start dates as the date I had applied for the skills assessment initial assessment and for completion I put the date that was on the certificate.
I haven't applied for the 190 state sponsored visa, I've applied for the 189, but from what I've read the easiest state to gain your license when you arrive in Oz is South Australia, so if I were you I would be looking to head to South Australia.
That being said its really hard to find a company to take you on and even harder to complete the Gap training, again from guys I know out there already, most end up as a trade assistants or don't even bother with plumbing as it seems companies only want licensed plumbers with Oz experience. The gap training issue it seems is that no one really knows what is needed so guys I know who have stayed plumbing have more or less had to do the full college course, which costs $Thousands!!!
Good luck with it all.