This and that are demonstrative pronouns: they are employed to ‘point at’ particular entities, usually to distinguish them from other entities.
In your first and fourth sentences, it is ‘expletive’—that is, it has no referent, it is present only to fill a syntactic ‘slot’ which would otherwise be intolerably empty. There is no entity pointed at, so it would be inappropriate to employ this or that. Only it is permitted to fill this ‘dummy’ role, just as only DO is permitted to fill the role of dummy auxiliary in questions. Any other pronoun raises the unanswerable question “What is its referent?”
That seems you are incompetent. ... What seems you are incompetent?
This is raining. ... What is raining?
In your other sentences, however, it is an ordinary personal pronoun, and it is possible to imagine circumstances under which it would be appropriate to employ a demonstrative.
I may say that your report is deeply appreciated, and your observations have been well received. As for your request, that is still under consideration; but I don’t think there will be any difficulty about it.
Respondents characterize the lemon drops and the peppermints as primarily “sharp”. The chocolate, however, is rated “sweet” when this is tasted.
Note that the ‘chocolate” sentence is awkward; the determiner (the) and the generic present tense (is) both struggle against use of the demonstrative. But it’s possible.