Yeah, it might be a solution - but I'm not sure if another one or two buttons will fit on the bar.

You can create arbitrary folder structures in Textastic. For example, you can create a local folder in Textastic for every FTP account. In the File Transfer screen make sure to select the target folder for the download on the left side before you start the download.

I actually already added this using the code at https://github.com/cncool/SlideSelection but then discarded it because it interfered with the standard keyboard gestures. For example, on the German virtual keyboard, you can swipe up on the "O" key to enter "ö". Also, holding the "o" key displays a row of accentuated characters like ó, ò, ô and so on - this also didn't work too well together with swipe selection. Also, it sometimes unintentionally entered characters when swiping over the keyboard.

I didn't find a good way to circumvent these problems.

I'll consider adding support for xml and html validation.

You can also add your vote for w3c validator support at http://feedback.textasticapp.com/topic/57678-w3c-validator-support/

Which syntax definition is selected in the File Properties popover?

I didn't have any of these problems in my test, but I'm going to try to reproduce it.

That would also be my suggestion: select the "XML" syntax definition in the File Properties popover. Textastic will use this syntax definition for all files with the same extension after you do that.

Can you please give me some more details so I can try to reproduce it?

Textastic doesn't include any compilers or interpreters. It can preview HTML, CSS, JavaScript and Markdown. You would have to upload the file to a server/computer and compile it there - for example using an SSH app like iSSH or Prompt. 

Long would be more like 10000 characters in a line. 200 characters is no problem. Yes, you can change the font size.