..he could have dwelt for a long time yet/still in that soft, well-upholstered hell, if this had not happened, this moment of complete hopelessness and despair and the tense moment when he had bent over the flowing water, ready to commit suicide.
Using yet there is a dated/literary alternative to what would normally be expressed as "he could still have dwelt for a long time in that soft, well-upholstered hell". As @200_success comments, it's a bad example for English learners.
That's not to imply yet should be avoided in general. Where it means [from some time in the past] up until now, it's perfectly normal ("I haven't finished writing this answer yet"). It's just not such a good choice these days where it references [some time in the future] after now, such as "I could yet/still finish soon". 1
1 Noting archaic/literary usages such as "I met him many years ago, when he was yet a young man", that should perhaps be "narrative reference time", rather than "now".
Many competent native speakers (and more particularly, writers) still use yet to reference a future time, so for them it's not really appropriate to say still is "better". But for learners I think it's probably easier (and "safer") to avoid yet in any context where either still or but would also be valid
The bold parts are the one that I have written, and I think the bold parts are the choices which you have advised me to write them,.
Other than that, I am wondering what the italic parts mean.