Is the "á" in casáu a proper Welsh letter? If so, why is it not offered a one of the options in the list of letters to use? Thank you.
Yes, Welsh uses several diacritics to modify pronunciation and emphasis.
Here the accent is used to indicate that the syllable with the a, the final one, is the one emphasised instead of the usual last but one syllable.
The character á is usually very easy to type with standard keyboard modifier keys - check on the web for how to do it on your system. For example, on macOS with the Welsh keyboard layout chosen it is 'alt-7, a'. On iOS, just keep any letter pressed down for a second to see a menu of its accented versions.
Is that why it wasn't included in the list of accents on the web version? Clicking on that would be quite easy