1) Header tags for homepage**, category, product pages is it required to** be have from h1 till h6? If No...then w3c validator will give me Warning and it will never Pass my site so?
The only requirement is a unique H1 on every page, the rest (h2, h3 and etc) if necessary.
2) Do google say h1, h2, h3..etc should be in sequence only? As per my site design I cannot define h1 always on top it can be after h2 or h3 so?
Not in sequence but properly nested. It means that H3 comes after H2, not H1. And H4 only after H3, not after H2 or H1. But you can use several h3's or h2's in a row.
Here's an example:
H1 Heading (My favorite cars)
** H2 subheading (Porsche 911) **
** H2 subheading (Nissan Nismo)**
** H3 (Nismo GT-R)**
** H3 (Nismo Z)**
** h4 (Z 350)**
** h4 (Z 370)**
** H3 (Nismo Juke)**
** H2 subheading (Mazdaspeed)**
** H3 (Mazdaspeed 3)**
** H3 (Mazdaspeed 6)**
** H2 subheading (Toyota AE-86) **
3) h1 can be only 1 for per page but it is fine h2, h3 etc multiple for same page?
as many as your structure requires as shown above.
4) At listing page to give all 30 products names as h3 is fine or it is not correct way?
If preceded by h2. if not, use H2.
5) For ecommerce site what is the best header tags you suggest? I mean till h3 is fine or h1 also fine?
H1 is mandatory. Next use H2, next H3 if necessary
6) Do header tags helps google in crawling ? or for ranking purpose? or header tags should be best for blog not for ecommerce site?
Yes, improves the crawl rate and lets google better understand the content of your page.
Thanks!