1). I am planning to enter the draw in 2019. Mine and my husband's income for last 2 years meet the requirements, this year would also meet it. But as we all know we will file the tax in March 2019, but i'll enter the dram in January. So in the draw do they ask for the income, if so, can i just put the rough income there? Cos later if i am lucky enough to get selected, by then i'll have the NOAs for 2018 too.
In my case, when my parents name came up in the lucky draw, I submitted their application after filing the income tax return for 2017. The result of draw was declared between March 19 - 21, 2018 and the deadline for submitting the application was May 25, 2018. Considering I had filed my taxes in February 2018, i had a lot of time to get the NOA from CRA. I will suggest you to do the same.
In case you are not able to file your taxes by the time lottery results are declared, you will also have an option to share your SIN# so that IRCC officer could retrieve your income information, but i believe it slows the processing of application. Also, in that case you will be required to fill form IMM 5516
2). I had another doubt, once you get approved as a sponsor, still you need to meet the income requirements until the parents become PR. Or is there any exception in that, cos i might go for maternity leave which would affect my income.
The sponsor has to maintain the required income till the Parents land in Canada as permanent resident. But please note that the EI benefit you will get during maternity leave will be considered as an income.
3). This could be little stupid, but is there any faster option for parents with only child. My parents are 62-63, I am their only child. So anything i could do if i don't get selected in lottery.
Its a valid question, I do understand your situation. Unfortunately the only option left (in case your parents's name doesn't come up in the lottery) is to apply for a super-visa, perhaps it's not going to match what a permanent residency offers.
Good Luck!