Your first quote is an instance of superlative of two which was fine for quite a while in English but started to be frowned upon in the 18th century, as M-W explains
This is an issue because in the second half of the 18th century some grammarians decided that it was unwholesome to use the superlative two; they concluded that one should say that something was the better of the two, and not the best.
The first author to warn against such use (Joseph Priestley) allowed that it wasn’t that big a deal (“a very pardonable oversight”), but by the end of the 18th century this idea was being written about as a rule, rather than a suggestion.
The fact that using superlatives of two was something that was quite common in English at that point, and had been for over a hundred years, seemed to matter little, if at all, to these grammarians.
We both doe burne alike
and frie with egall flame:
But I am weakest of the two,
my nature willes the same.
— George Tuberville, The Heroycall Epistles Of the Learned Poet Publius Ouidius Naso, In Englishe Verse, 1567
However, even today, the superlative of two still survives and is a matter of... opinion, says the same post:
In spite of continuous and widespread use, and occasional protests by people who studied language, the prohibition on the superlative of two has remained something that usage guides warn against. Our Dictionary of English Usage refers to this as
a perfect shibboleth, serving no practical function except to separate those who observe the rule from those who do not.
You may, if you wish, refer to something as the best of the pair, rather than the better, and rest secure in the knowledge that the only rules you are violating are those of usage (read: opinions), and not of grammar (the structure of the language). Bear in mind that this use will quite possibly annoy some portion of your audience, but it doesn't hurt to go with the option that you think works best.
Grammarphobia confirms that the superlative of the two can still be encountered in everyday usage:
Since the late 18th century, the convention has been to use the comparative (the intermediate degree of comparison) when two things are being compared, and the superlative (the extreme degree of comparison) for three or more. However, the so-called “superlative of two” – as in “she’s the oldest” when there are only two people – has a long history and is common in everyday usage.
The fact that it is not easy to agree on this is well expressed in this EL&U discussion: Use of the superlative when only two items are present.
As for your second quote, it is clearly written in the perspective of the main character, which is Emma, and so it is not surprising that in her own thoughts, Emma would refer to herself as herself. You can rewrite that quote in direct speech as follows:
She (Isabella) has been a friend and companion to me (Emma) such as few possess: intelligent, well-informed, useful, gentle, knowing all the ways of the family, interested in all its concerns, and peculiarly interested in myself (Emma).
About the usage of the reflexive pronoun as the object of a verb instead of the personal pronoun in the accusative, you can read this usage note (Collins) that has been quoted again and again in EL&U different posts:
There is ample precedent, going as far back as Chaucer and running through the whole range of British and American literature and other serious formal writing, for all these uses.
