Please create another screencast. I must be doing something different. It is very annoying indeed. Sorry for that.

Are you absolutely sure you are using the latest version 1.1? You can check in the About screen. The new version also has tabs.

When I try to follow your steps it works without problems...

It's possible to add custom TextMate-compatible syntax definitions to Textastic:
http://www.textasticapp.com/v4/manual/lessons/How_can_I_add_my_own_syntax_definitions__themes_and_templates.html

Unfortunately, I couldn't find a TextMate bundle for "Mplus", sorry. Maybe you can find one.

Usually a google search for "<programming language> tmbundle" brings up results if there is a syntax definition for that programming language.

Since Auto Save is now the default for modern NSDocument-based in Mac OS X, I think Transmit should add an option to support this better.

As I wrote above, if I disabled Auto Save, other features like Versions and iCloud would be disabled, too.

Currently, you can't, but an option for hiding the tab bar when only one file is open in a window is on my todo list.

There seems to be a problem when parsing the directory listing returned by your server.

Please try to enable the "DebugLog" setting in the connection's settings screen. Tap on the blue arrow on the right side of a connection to change its settings. This creates a log file when you try to connect to the server which might help you find out what's wrong.

You can send me the log file to support@textasticapp.com - please remove your password/user name from the log file if you don't want me to see it.

This was actually a change in Mountain Lion for apps that support iCloud. Other iCloud-enabled apps like Apple's TextEdit, Pages or Numbers or third party apps like Byword and iAWriter have the same behavior. 

Here's the relevant part of the Mountain Lion AppKit Release Notes (https://developer.apple.com/library/mac/#releasenotes/Cocoa/AppKit.html#//apple_ref/doc/uid/TP30000741):

NSDocument Support for iCloud
In 10.8, NSDocument-based applications with a ubiquity-container-identifiers entitlement gain new functionality and UI to facilitate iCloud document management.
When iCloud is enabled and an application is first launched or re-activated and no windows are visible or being restored, instead of creating a new Untitled document, NSDocumentController will display a non-modal open panel showing the user's iCloud library. From there an application can either open an existing document from iCloud or the local file system, or create a new document. 

You can press Cmd-N to create a new document even when the open dialog is shown.

There's currently no auto-format functionality in Textastic.