These seats are reserved for ladies. The old lady running the roadside lunch house brought him rice and curry. The new District Collector is a lady. Calling somebody a 'woman' is usually perceived as rude here, with the notable exceptions of news journalism, law, bureaucratese and scientific literature: Bangalore woman wins Maths Olympiad.
A lady or a woman? - English Language & Usage Stack Exchange
The plural possessive is "ladies'." "Lady" is singular, so if you were referring solely to one woman's shoes, it would be "the lady's shoes." As for your second question, I'm assuming you're referring to a group of women in your salutation of them, so it would be "Good morning, ladies." And as you're addressing them directly, the comma preceding "ladies" is necessary.
Idiomatically, it is gentleman. Lady comes from an Old English compound noun meaning roughly "loaf kneader," whereas lord comes from a compound noun meaning "loaf keeper" or "loaf protector." The etymological counterpart of gentleman, which is indeed gentlewoman, is used infrequently these days, usually in historical or quasi-historical contexts.
single word requests - Is there an opposite gender for "lady ...
Lady Macbeth wants to substitute her milk (which would nourish a kid) for gall, which today would mean boldness and impertinence, but also refers to bile (Merriam-Webster). So here, too, the milk is too kind, too nourishing, and so it must be switched for something more murderous so that she can physically and mentally go through with murder.
Ladies is the plural form of lady, so the apostrophe goes to the right - ladies'. If you are wondering why we don't write ladies's, it is because ladies is one of the exceptions, along with girls', parents', players', weeks' and even Klingons' It can get a bit niggly with names too. Aristophanes' plays, but Jesus's miracles and (usually) James ...