(1) Were these wheels available as a 240 option as late as 1990?

Depends what you mean by "option". Corona rims were never officially an option on 240. That is, you could not check a box and order a car equipped with Coronas. Certain models/years came with Corona wheels as standard equipment. If the particular model you wanted did not come with them, and you wanted them, the original selling dealer would just take 4 "accessory" Corona rims from the parts department and mount them on the car you were purchasing.

In that sense, Corona rims are still to this day a dealer installed accessory. You can still march in and order them. In fact, there are still two valid part numbers - one for '75-85, and another for '86-'93.
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(2) Also, when were the original "small" hub covers (approx. 3 cm in diameter) replaced by the larger aluminum hub covers (approximately 20 cm in diameter)?

I believe all cars factory delivered with Coronas bore the large caps. Again, if you got the rims dealer-installed, you had your choice.
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(2a) I have noticed that the wheels have cast letters (country of origin, Norway) near the center that would not be covered by the small hubs.

True indeed. I have in my fleet an '86 244DL wearing Coronas and small caps. The lettering is unnoticeable from more than 0.5m away.
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(3) Was this the case for early corona wheels that were used with the small hubs.
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Yes. Every wheel is embossed with the country of origin, wheel dimensions, and part number. But your question bears an incorrect fact. Every Corona wheel could be used with a small cap. To the best of my knowledge, no car ever came off the boat bearing Coronas with small caps. Early or late, it makes no difference. The only difference among all the Coronas is that the correct wheel for 86+ applications had a hole for the locating pin. Other than that, there is no concept of "early" or "late".

Hope this helps,

Evan