Learn English – Is a thumb also a finger

You're an aspiring novelist? If so, we have that much in common. In any case, on to answering the question:

'Hold' is a rather vague, general verb. You have an opportunity to give the reader some insight into Mary's emotional state simply by choosing the appropriate verb. Does she caress her finger (fondly) if its to be removed and this is perhaps the last time she'll be able to touch it? It it painful, and so she grips it or her hand clamps over it? As hinted by the last example, a phrasal verb may be appropriate too, if its more revealing and in line with your vision. So you need to ask yourself, 'Why is Mary holding her finger?'

If you decide to use 'hold', I don't think the prepositions you've included are necessary. There's no need for either 'on' or 'onto' in this context. They're only additional words that add bulk to the sentence. Incidentally, all three sentences mean the same thing, as far as I can see.

If there's some significance in Mary's touching her finger (which there should be if you've thought to include it), then maybe you want to have that instance stand out, which would be better achieved by breaking up the sentence.

So: Mary peered down at the beach. She 'held' her finger and sighed, sinking back to her seat.

Having written the above sentence, it seems you could consider bring the 'sigh' closer to the act of 'holding' her finger. The two are connected, and if you include the fact that she sighed, you might not need a more specific word for 'held'.

Neither is a common phrase, but we can make a good guess from context.

By implication, cracking his thumb (I'm guessing in the same sense as "cracking your fingers") is an insulting gesture in the speaker's culture.

"Black be its fall" is an idiomatic version of "may its fall be black", and again by implication is intended as a wish that the House of Shaws shall fall in an unpleasant manner.

You didn't tell us where this came from, but I'm guessing (from the use of "laird") that this is intended to be a Scots dialect or something related to it. If you want to research the exact meaning of these phrases, that may help you do so.

(@FumbleFingers' guess that these may have been made up is a good one. It was common for authors to invent "bad language" -- curse words and the like -- to avoid printing something that might offend the reader. We still see this in television scripts, such as the invention of "frack" in Battlestar Galactica. It's also common when you want something that seems appropriate for the speaker's culture but aren't willing or able to do the research to find the correct phrase -- lazy, but common.)