“Nous sommes assurés” is in the passive voice. The same construction exists in English: “we are assured that …”. In French, the sentence could be rephrased using the indeterminate pronoun on: “puisqu'on nous assure que …”. The meaning would be approximately the same, although using the passive voice reduces the emphasis on the fact that someone has given this assurance: “nous sommes assurés” can be read as a state of being — “nous sommes” = “we are”, and “assurés que …” is a state of mind.
“Nous sommes assurés” is in the present tense. The rest of the sentence happens “dans peu d'années” (“in few years”), hence the future tense. The last part “qu'elle se soit enfuie” is a subjunctive past, with the past tense indicating that the action in the relative clause happens in the past compared to the clause that it is a complement of, i.e. “qu'elle se soit enfuie” is in the past compared to “regretter” which itself is at the same time as “nous ne penserons à elle”.
Thus:
Dans peu d'années, nous ne penserons à elle que pour regretter qu'elle se soit enfuie.
In few years, we will think of it only to regret that it had escaped.
Nous sommes assurés que dans peu d'années, nous penserons …
We are assured that in few years, we will think …
And elle refers to “notre petite vie quotidienne, qui nous semble bien un peu morose, certes, mais qui est, sans doute, charmante”. Without the adverbs that qualify the impressions we have of this day-to-day life, that's “notre petite vie quotidienne, qui nous semble un peu morose, mais qui est charmante”.