Cropping text does quite go against this. One of the basic tenets of Scribus was to render text as well as possible. Scribus always tries to render text as fully as it can, meaning that if it can't fully display a character it will not display it at all. Scribus has no cropping functionality for text. I assume you want to do something similar. My attached image shows - from top to bottom reading downwards - (a) original text, (b) text cropped top and bottom, (c) text cropped by a shape. If you are experimenting then it might be good to put that specific piece of text into Inkscape first and then export different manipulated versions into Scribus until you get what you want but Scribus has plenty of vector tools so that's not usually necessary. Unfortunately you won't be able to edit the text after converting to outlines so keep a backup of the original text if you need to try different things. To skew text you will have to convert it to outlines and then skew the resulting shapes.

On your second question, you can skew the frame - by either using the Node Editor or via menu 'Item -> Transform' - but the text will not be skewed along with the frame. The 'clipping' refers to the image rather than the text. It's like a contour line but it's part of the image file and not a Scribus thing.

The clipping path of an image is only really used to control how text flows around that image. I know you said it was a peculiar case but is there an example you could supply that would show people what you want? Even a simple scanned hand-drawn picture might help. If you don't want the letters then it's usual to just not add them in the first place. I don't understand why you would want some of the letters to go missing. For the first of your questions, I'm not sure what you want to do.

Scribus text flow around image