How to Optimize eCommerce website ?

You can check this article on how to optimize ecommerce website and it's issues like you mentioned few above and in general.
Here are few tips to optimize your website.
1.Keyword Research
2. Minimize CSS and Inline styles
3. Use CDN for faster loading of website
4. Competitor Analysis
5. Content Marketing
6. Blogging
7.On-Page optimization
8.Check on Woorank, Seositecheckup to know more issues in the website.

For full detailed anaysis of each and every aspect mentioned above please go thorugh this article by one of the top notch anayltics company KissMetrics .
https://blog.kissmetrics.com/seo-for...erce-websites/

The following is some ways helping you optimize your eCommerce site:
- Control duplicate content
- Quality over quantity
- Get cutting edge with Google Adwords
- Explore all types of content
- Link with social media to promote your site
- Competitor analysis
- Implement email marketing stategy

First of all, which platform are you using for your website. The following issues occurs mostly for custom development without re-checking coding validation.

1. Inline Css
Ans: As I have mentioned, inline CSS needs to be replaced with ID/Class and go for external CSS to execute styles.

2. H2 Maximizer
Ans: Not sure whats the current situation of your website, could you PM the detail report please.

3. Http Request
Ans: You might have already known that minimizing Http request is very important to improve website speed/performance. This can be done through several ways:
* Optimizing images
* Use CDN
* Use Font icons instead of image icons
* Combine CSS and JS for less Http requests
* Reduce scripts and put them at the bottom of the page.
and many more.

4. CSS not minified
Ans: Can be done through combining multiple CSS files, minified them (there are many online minification converter available), if you are using a CMS then suitable cache plugins are available which comes with minification feature.

5. 40 Error
Ans: Please specify the errors.

6. Non follow links for index related .
Ans: Can be solved through SEO plugin of your CMS.

Thank you

How to fix them?

Use Shopify.

Good luck with your venture.