En français contemporain. “Il en est du mariage comme des oiseaux en cage : ceux qui sont en dehors désespèrent d’y entrer ; ceux qui sont au-dedans veulent à tout prix en sortir.”

For your first question, d’un pareil soin.

Soin has an old meaning, almost never used today, that means worry, anxiety. So, it means that the birds outside the cage suffer from the same pain as the birds inside, the firsts wanting to go in, the seconds to go out.

“D’un pareil soin” is a turn from the old French language and it would not be used nowadays. We could find in a contemporary translation something like this, in which the idea is not explicit as the expression has been simply omitted;

• … Le fait qu’il s’en voit si peu de bons est un signe de prix et de valeur.
  Il en advient comme aux cages: les oiseaux qui sont dehors cherchent désespérément à y entrer, et ceux qui sont dedans cherchent tout autant à en sortir.

“D’un pareil soin” signifies “in reason of the the same state affairs”, this implicitly stated state of affairs being the imprisonnement by means of a cage, which is also a means of isolation of the “prisoners” from the outside world. Interestingly enough, there is a chinese proverb that rhymes strangely with Montaigne’s thought, and that I think worthwhile mentioning;

• Le mariage est comme une place assiégée ; ceux qui sont dehors veulent y entrer et ceux qui sont dedans veulent en sortir.

As per Littré, “advient” comes from the initial form of “advenir” (avenir) and this verb meant “échoir”, “se faire”; “se faire” can be translated as “to come about”.