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At the top of the file navigation hierarchy in Textastic you can choose between "Local Files" and "iCloud".

Files in "Local Files" are stored on the local file system and are not automatically synced.

The "iCloud" option corresponds to the "Textastic" folder/container in iCloud Drive. Currently, you cannot send files to another iCloud Drive folder from within Textastic.

You can use the "Open…" command and choose "iCloud Drive" to open a file from any folder in iCloud Drive and edit the file in-place. Files opened in this way do have some limitations though: through the iOS sandboxing system, Textastic only gets access to this single file and not to the whole folder. So, if you do a web preview for example, the web view cannot access other referenced files like images or CSS files in the same folder.
To bring up the keyboard, you need to do a single tap on the text. If you do a double-tap before the keyboard is up, it will only select the text so you can copy or cut it. This is consistent with Apple's Notes app, for example.
I'm currently working with the developer of Working Copy to improve the experience. In Working Copy, you will be able to choose "Edit in Textastic" to open a file from Working Copy in Textastic. You can then make changes in Textastic and choose "Save in Working Copy" in the Textastic action menu which will save back your changes to Working Copy and ask if you want to commit.

In Textastic, files opened like that will have file names like "filename_wc#1A2B3.txt" and will be under Local Files in the Working Copy subfolder. You can upload those files to remote servers and the file will automatically be uploaded as "filename.txt".

With iOS 9 multi-tasking on iPad you can then open Working Copy as a Slide Over which makes it pretty convenient to edit several files in the same repository using Textastic.
Please try to reboot your device. It fixed a similar issue for me.
Hello, can you please try to reboot your devices? Sometimes it can fix issues like that (unfortunately).
I don't think I have changed the Sublime Text .tmlanguage file when packaging it for Textastic. It should work without changes. If it doesn't, I'd say that this is a bug in the .tmlanguage file...
You can add custom syntax definitions to Textastic by using TextMate bundles. SeeYou should be able to use the TextMate bundle you linked to: https://github.com/tbuser/openscad.tmbundle