Apologies for the delay in replying. The theme we use is not 'responsive' in that it does not alter the site for mobile users. The theme actually renders very well in mobile devices without doing so which is a good reason to continue using it.
1) My preference is to work in this way because then the user gets to see the things I want them to see. With 'responsive' themes and sites it is common that mobile users do not see banners and other important aspects of the site. If the purpose of the site is to make money then it is better to have a site that carries the monetisation for ALL visitors even if a few people don't like it than it is to have a site that everyone can see easily, no matter how bad their mobile browser is, but where there is no way to earn money from that subset of visitors.
Of course you can change the theme yourself at any time in the future if, for example, you find that a majority of your visitors are using mobile phones to see your site AND they are spending money on the site.
2) I have never purchased a Nichejet site but they seem to be concentrated upon providing a basic Adsense site. We are not big into Adsense.
3) I suggest that we leave everything in. It is the work of moments to delete something that you find you don't want and will take ages to research and re-add something we delete that you find you later want. Tags are a very useful way to provide search engines and humans with a way to explore the site and that is why we use them. You can delete the tags and tagging very easily if after exploring you find your opinion is unchanged.
4) Yes you can promote as many or as few products as you wish. We provide the tools to enable you to manage the site as a business unit. You choose how to use the tools.
Yes, taking these sites and learning how and why they operate as they do is a good strategy which is why I recommend it.
The chances are that even when you are familiar with the hows and whys that it is still cost effective for us to build the sites for you - we have clients for whom we have made literally hundreds of sites and that is the reason why.