How did "lady" and "ladies" come to differ in conveying degree of respect?

The respectful address for one lady is "Madam" or "Ma'am" -- as in "Ma'am, are you ready to order?"

But you do not ever address a group of ladies "madams" -- because if could sound like your are calling them brothel-owners. You also do not call them ma'am-s. (It does not seem suitable to make plural). So-- you say ""Ladies, are you ready to order?" and with repetition this seem even more respectful.

Since no one (normally) address one a "Lady" (you either know their name or you say ma'am), to address one as "Lady" comes across as forced or weak effort to be respectful... such as when your patience is coming to an end.

But it still seems (to me) one can still call a 3rd party a lady in a very positive way, such as "she was a such a sweet young lady."

"Lady" is never used to address someone by itself. It can be used as either a title for nobility or as a common noun, but it is always meant to be used alongside an identifier, such as a name or article. For example:

Lady Macbeth threw a knife at the old nun.

or

The lady caught a knife that was thrown at her.

However, using the term "Lady" without such an identifier, such as the name of the noble lady or an article to signify which lady you were speaking of, leaves the phrase incomplete. Hence it sounds disrespectful not because of the context, but because of its incompleteness.

On the other hand, since "Ladies" is a plural term, it already encompasses an entire group of people without the use of an identifier.

For your example, the correct form of usage in that sentence would be either of:

My lady, are you ready to order?

or

Lady Macbeth, are you ready to order?

You can also reorder the sentence to get:

Is the lady ready to order?

All of these are equally respectful, though "My lady" also has a context of subservience.