I use mail via the JetPack plugin to submit drafts of content - which is very convenient because I live out of my inbox. Response from clients on using this option have also been very positive because of the very low user threshold. Everybody can send an email.

I have also used MS Word, which support the XML-RPC interface for multiple accounts. Clients have been positive about this option as well. It's an environment they are familiar with, which makes all the difference.

Lately, I have been using Evernote more and more - and the ability to publish via mail from a note in Evernote is nice. The advert in the bottom of the mail is annoying, but is easily removed before publishing in Wordpress after adjusting some SEO, social media distribution and other items. Evernote handles both online and offline editing through syncing, which is close to your specifications.

There used to be a plugin called EverPress - to autopost from a shared notebook in Evernote. But it seems it is no longer maintained. Perhaps this could be a viable alternative - to update this plugin?