I'm testing an upgrade to 5.9.1 and have installed mosaico (versions listed below) and have a few issues with mailings.
Try to create a new mailing. I don't see the standard HTML window with the wysiwyg editor. I see one for Design which has the mosiaco templates in it.
If I select a standard (non-mosaico) template, I don't have anyway to see it nor edit it.
If I select a mosaico template, I get that editor and can edit it no problem.
Another issue that may be a clue, is if I hit preview HTML either from the main screen or from within the mosaico editor, nothing happens. I get a db error with FK constraint on civicrm_mailing_trackable_url. The sql is doing an insert and doesn't have the mail id. It is only trying to insert url.
Clearly this works else where so I must have something basic wrong.
Shoreditch 0.1-alpha30 I could never get this work. It says it's optional so I skipped it. In playing with this, the not working could be related. It might work, in the mailing screens I wasn't getting the options for regular template and it looked like things weren't necessarily in the right place.