'Sagaseba sagasu hodo' means, basically, 'the more I search....', assuming that it is in fact you that's the subject. You don't want to use it here.

'Ikura sagashitemo mitsukaranai' is a natural way of saying this in Japanese. The Japanese often use the intransitive verb 'mitsukaru' where we in English would use the transitive verb 'to find (something)'. Sorry for the romaji, haven't got my Japanese input up yet.

This is the best explanation out there, courtesy of Shakin' Stevens. https://youtu.be/JjHwMiLSiqA

Often when we say the world 'old' we don't actually pronounce the 'd' on the end. This spelling is just an attempt to represent the fact that this consonant is being dropped. In written English it adds a colloquial, folksy charm, at least in theory....

The context is not very clear even with the extract you've given. Nonetheless, it means 'it' (some new way of living?) makes the old ways (of living?) seem intolerable, as the old ways are based on, are dependent on, or rely on, a pretence, which is to say, an act of pretending, on the part of the person in question. I hope this explanation is not even more difficult to understand than the original sentence! Udaci!

It is not slang, and not a mistake. It is a very widespread non-standard grammatical form, associated most commonly with relatively uneducated working-class people. You will hear this type of usage in various parts of the UK as well. When I saw the title of this post, I actually assumed it was a Cockney character speaking this way, which should indicate how widespread the usage is. Don't make the mistake of thinking that the standard English you learn as non-natives is some pure, correct, version of English. There are all sorts of non-standard grammatical items in different dialects that help make English so interesting. Think of the 'youse' of the Irish, the 'it were fantastic' of Yorkshire, or the 'I already done it' of the Cockney. Having said of all which, students of English should certainly not try and speak like this, even in jest. I hear this as dialect because the speaker is clearly a native speaker, with an accent and a manner of speaking that mark the non-standard usage as being part of a local dialect of some sort. If you speak like this as a student, people will hear what you say as being wrong, period. Observe, remember, but do not repeat.

Of course, this is literature. That means that word use is more creative and open to interpretation. HOWEVER, to round something means to knock off the edges, to polish it, to make it round and smooth, to finish it like a woodworker or stonemason would finish a piece of wood or stone. What I think it means here is that friendships are deepening, becoming more polished, more complete, as people spend time together under the circumstances the author is describing.

It sounds natural enough to me....