Learn English – She has bought some cloth.She will make/is going to make herself a blouse

BE + going to - Lindsay is going to fly to New York next week.

Forms with BE + going to possibly originated in such utterances as:

1. We are going to meet Andrea at the cinema,

uttered when we were literally going, i.e. on the way, to the meeting. At the moment of speaking there was present evidence of the future meeting. This use has become extended to embrace any action for which there is present evidence – things do not have to be literally moving. Consider now these two utterances:

2. Look at those black clouds. It's going to rain.
3. Luke is going to see Bob Dylan in concert next year

In [2] the present evidence is clear – the black clouds. In [3], the present evidence may be the tickets for the concert that the speaker has seen on Luke’s desk, or it may simply be the knowledge in the speaker's mind that s/he has somehow acquired.

Modal (will) - Lindsay will fly to New York next week.

Will is a modal and, like the other modals, has two core meanings. The two core meanings for most modal are:

(a) the 'extrinsic' meaning, referring to the probability of the event/state (b) the 'intrinsic' meaning, reflecting such concepts as: ability, necessity, obligation, necessity, permission, possibility, volition, etc.

The extrinsic meaning of will is exemplified in:

4. Emma left three hours ago, so she will be in Manchester by now.
5. There will be hotels on the moon within the next 50 years.
6. The afternoon will be bright and sunny, though there may be rain in the north.

In all three examples, the speaker suggests 100% probability, i.e. absolute certainty. (MAY would imply possibility, MUST logical certainty, to take examples of two other modals). Note that while certainty in [5] and [6] is about the future, in [4] it is about the present. It is the absolute certainty, in the minds of speaker/writer and listener/reader, that can give the impression that forms using ‘the will future’ are some way of presenting ‘the future as fact’. Some writers therefore call this form ‘the Future Simple’. Weather forecasters, writers of business/scientific reports, deliverers of presentations, etc, frequently use will, and learners who encounter English more through reading native writers than hearing native speakers informally may assume that it is a 'neutral' or 'formal' future. In fact the particular native writer or speaker is simply opting to stress certainty rather than arrangement, plan or present evidence.

The intrinsic meaning of will is exemplified in:

7. I'll carry your bag for you.
8. Will you drive me to the airport, please?
9. Jed will leave his mobile switched on in meetings. It's so annoying when it rings.

These examples show what we might loosely call volition, the willingness or determination of the subject of the modal to carry out the action. Note that [9] is not about the future, and in [7] and [8] the futurity is incidental. It is context rather than words which gives the meaning.

It seems possible that there is some confusion here between have used as an auxiliary verb, and have used as a standalone verb. You say that you "choose has because because it denotes the action that he has a mill in the past and he is still having it." That would be the case if the sentence were

He has owned a mill.

However, in this case, have is a standalone verb indicating possession. Just replace it with possess or own and you will understand the correct usage. So, yes, (b) is correct, but perhaps not for the reason that you suggested. In this case, has is simple present, indicating that this individual currently owns a mill. Other examples of have conjugated as a verb in its own right:

He had a mill...but he may not any longer

He has had a mill...for a while now.

Note that the second sentence is what you were talking about: he had a mill in the past, and he still has it. As well, we do not usually say that he "is still having it." When it comes to verbs that refer to states, not actions, the present is generally preferred over the present continuous. By contrast, as you probably are aware, the present continuous is ubiquitous when it comes to current actions, and in such cases substituting the simple present may well imply habit rather than current behavior: I eat vs. I am eating.