I already solved the problem. When I tried to create the project with the current version of yeoman and in this version yeoman put an old version of gulp in the project. When he tried to import react 16, the old version of this gulp requested react in the wrong way. I solved this issue by developing my project into an existing spfx sample project ...
Short answer to your question is Yes.
SPFx Extensions Samples.
SPFx List View Command Set Example using React ...
As per my understanding, SPFx webparts are responsive by design. So, most of the times you don't need to use bootstrap or media query.
It is recommended to use Office UI Fabric React components (not compulsory) which is a responsive and simple for you to create web experiences by using the Office Design Language. Read more at Design considerations for ...
You should check if, in the settings of the group, the Who can view the membership of the group? is set to Everyone. It is probably set to Group Members so you should change it to Everyone.
You can check that setting by going to https://SITEURL/_layouts/15/user.aspx , then clicking on group name and choosing the menu item Settings -> Group settings.