As a speaker of US English, I am nonplussed by your version [i]; I have only encountered [ii].
The only circumstance in which I could imagine [i] occurring is an ‘echoic’ denial of the positive proposition:
A: He’d better tell them.
B: No, he hadn’t better tell them. He’d better keep his mouth shut.
But the idiom may be used differently in the British English which is CGEL’s underlying standard.
Wishes are unrealizable conditionals, so we backshift the tense of the verb. can is backshifted to could, will to would, and present simple meet is backshifted to simple past met. be is an exception because it has a proper subjunctive: am/are/is backshift to were.
You mention guidelines about action words for future actions, but the word now in examples a and b locates the sentences in the present, so these guidelines don't apply. Furthermore, these guidelines explain what you should do, but they don't explain why you should do it.
I believe that it is much more effective to go back to first principles and make a complete sentence that expresses exactly your wish as it can happen or is/will be happening, then backshift:
I am eating that cheesecake
I wish I were eating that cheesecake.
I can eat that cheesecake
I wish I could eat that cheesecake
My father will let me borrow the car
I wish my father would let me borrow the car
Note that eat is an action word, but the first two examples relate to the present so the "action verb" rule doesn't apply. The first example does not require could and the second example does. Working this way, you can see why could is necessary- because can expresses ability / permission, and can -> could. The third example is about a future event, so will -> would.
You are right that a) and c) are correct. To find out what's wrong with b) and d), remove the backshifts and see what you get:
b) I meet her now.
d) I meet her more often.
b) doesn't make sense because meet is present simple, and we don't use present simple for current events- we use present continuous. To find the correct way of talking about now, change that sentence to present continuous and backshift it:
b) I am meeting her now
b) I wish I were meeting her now.
d) doesn't make sense because more often requires some reference level. If you talk about capability, there is an impled reference level:
d) I can see her more often [than I do]
d) I wish I could see her more often [than I do]
Best Answer
Had better in [had better + bare infinitive] can be seen as a two-word item that functions as a modal verb, meaning essentially should or ought to. (At least in terms of traditional grammar; modern grammars likely better classify and/or describe this.)
What seem like past forms (preterites) in modals do not necessarily confer a past meaning on the infinitives they modify. An example of this is Could you help me? Although could can be used as the past tense of can, it does not indicate any sense of the past in this example.
A deep explanation as to why this is so relies on understanding the evolution of English at a fairly sophisticated level, which is beyond my knowledge.
Should better is not normally used in Standard English, or if it occurs, is probably uttered by some speakers informally and may have arisen as a sort of fusion of should and [had] better.
The following Wikipedia article, in somewhat fragmented form, addresses some of these issues, and includes a subsection specifically on had better and ought to.
https://en.wikipedia.org/wiki/English_modal_verbs