Learn English – Usage of the definite articles with personal names

Your description of the use of the definite article as opposed to the indefinite article is accurate. The presence or absence of one of these articles, however, in no way alters that standard usage.

As for why the article may disappear, it strikes me that when the article is there, it draws attention towards the adjective and away from the person. Let me use sentences 4 and 5 to illustrate:

It seemed Walter didn't pay any attention to a tearful Kitty.

Stated this way, the sentence indicates that Walter is not paying attention to Kitty's tearfulness; he is not moved or affected by her distress.

She hasn't got lovely eyelashes like marvelous Monica.

In this case, the attention of the sentence is squarely on Monica, and "marvelous" is relegated to being a pure adjective, neither calling for nor receiving any unusual or special notice from the speaker. If we added "the," it would put the attention on the fact that Monica is marvelous. (This can, by the way, be a vehicle for sarcasm, if you want to point out that she's really not all that marvelous after all.)

While I wouldn't dare to say that this is a 'rule', I would say that the definite article the is generally used before mass / uncountable nouns when you are referring to a specific instance of that noun.

In your example, you are referring to the Inventory and stock list amount, rather than, say, the purchase amount, or the sales amount, or the amount of your salary.

Similarly, you might say:

The twenty people on the course - as distinct from people in general (no article), or the millions of people not on the course.
The hospital staff - as distinct from The university staff
The river water - as distinct from water in general (no article), or from The lake water.