Hi! Here are some answers to your questions:
- Customisation of robots.txt and htaccess files
I know you auto-create a sitemap.xml; I want to omit pages from the sitemap.
Here is how to add the robots.txt: Add a robots.txt file - Sitejet Help
There is no support for .htaccess - you can hide pages from google with this option: SEO with Sitejet - Sitejet Help
- adding alt tags
Yes. Add an Image Element and set the “Alternative text” property
- 301 redirects
Yes. Read more about redirects here: Redirects - Sitejet Help
- applying canonical
Again, I know you do this automatically. I am asking about a manual method
I’m not aware of a manual option.
- fast migration of blog articles (possibly using an RSS feed).
There is an import CVS option for Collections (Blog is a Collection), read more here: Collections - Sitejet Help
- add schema
There is no built-in schema, you need to add it yourself, read more about this on this thread: Schema.org for Sitejet - #8 by Lucian_Dinu
- add href lang tags
I’m not aware of these href lang tags.
- password protect individual pages
Yes, you can read more here: Password protected pages - Sitejet Help
Hope this helps!