What does the bold part function in grammar? What is the structure?

Question

Machiavelli's "The Prince" Chapter XVIII

W. Marriott's translation:

Therefore it is unnecessary for a prince to have all the good qualities I have enumerated, but it is very necessary to appear to have them. And I shall dare to say this also, that to have them and always to observe them is injurious, and that to appear to have them is useful; to appear merciful, faithful, humane, religious, upright, and to be so, but with a mind so framed that should you require not to be so, you may be able and know how to change to the opposite.

There are only non-finite forms without a subject nor a verb.

Does all the bold part function as the subject? Thus, "{To ..., and to ..., but with ...} is useful"?

H. Mansfield's translation:

Thus, it is not necessary for a prince to have all the above-mentioned qualities in fact, but it is indeed necessary to appear to have them. Nay, I dare say this, that by having them and always observing them, they are harmful; and by appearing to have them, they are useful, as it is to appear merciful, faithful, humane, honest, and religious, and to be so; but to remain with a spirit built so that, if you need not to be those things, you are able and know how to change to the contrary.

The same question.

"as it is {but to ...}"?

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I have no problem with the two other English translations.

N. Thomson's one:

It is not essential, then, that a Prince should have all the good qualities which I have enumerated above, but it is most essential that he should seem to have them; I will even venture to affirm that if he has and invariably practises them all, they are hurtful, whereas the appearance of having them is useful. Thus, it is well to seem merciful, faithful, humane, religious, and upright, and also to be so; but the mind should remain so balanced that were it needful not to be so, you should be able and know how to change to the contrary.

J. Bennett's one:

So a prince needn’t •have all the good qualities I have listed [on page 33], but he does need to •appear to have them. And I go this far: to have those qualities and always act by them is injurious, and to appear to have them is useful—i.e. to •appear to be (3) merciful, (4) trustworthy, (6) friendly, (8) straightforward, (11) devout, and to •be so, while being mentally prepared to switch any virtue off if that will serve your purposes.

Answer 1

... ;to appear merciful, faithful, humane, religious, upright, and to be so, but with a mind so framed that should you require not to be so, you may be able and know how to change to the opposite.

is a disjunct compound non-finite clause that elaborates upon what was meant by "to appear to have them". Repunctuation might make it clearer:

Therefore it is unnecessary for a prince to have all the good qualities I have enumerated, but it is very necessary to appear to have them. And I shall dare to say this also, that to have them and always to observe them is injurious, and that to appear to have them is useful—to appear merciful, faithful, humane, religious, upright, and to be so but with a mind so framed that should you require not to be so, you may be able, and know how, to change to the opposite.

There is a danger in always being merciful, faithful, humane, religious, and upright. You can act so sometimes, or even most of the time, but you need to have the cast of mind that will allow you to be umerciful, unfaithful, inhumane, irreligious and devious now and then, as needed.

to appear to have them. Infinitival clauses can serve nominally as the subject of a verb: To err is human, to forgive, divine.

So, "{to appear {to have them} } is useful".

We could also say "appearing to have them is useful", and could restate the bolded clauses using the -ing form:

... and that appearing to have them is useful—appearing merciful, faithful, humane, religious, upright, and being so but with a mind so framed that should you require not to be so, you may be able, and know how, to change to the opposite.

Thus, you could understand the compound infinitival clauses

to appear merficul ...
and
to be so but with ...

as a nominal standing more-or-less in apposition to "to appear to have them".

Depending on how broad or narrow your definition of apposition is, of course. If you insist that only bona-fide nouns can stand in apposition, then not. But if you allow for any construction that functions nominally, then yes.