Hi,
You have written 'Tom loves ONLY Mona', but in this case 'only' tell us something about the verb love and not on Mona. I justify my mind, Tom can love one woman like Mona in your example but Tom can love two or more girls, too. In conclusion the adverb 'only' give us information about how many persons Tom loves.
We can rewrite your sentence like that : Tom loves not more that one girl, she's Mona.
The word 'only' doesn't modify the proper name 'Mona'.
But I am just a learner and I'm sure you follow the way of my mind.;-)
I have a break it's time to lunch (8 p.m).
Have a nice and sunny day (Note: I'm interested about the name of the book in which you have read that adverbs can sometimes complete the nouns).
Best,
The Frenchie.:-D