They had placed their rifles and helmets in a corner and waited for supper, as well behaved as children on a school bench
and
They entered covered with a creamy looking froth that gave them the appearance of meringues
Creates visual interest to text elements is alignment
APA style. it's most used for citation in research papers
Yes, it seems to be an alternative, in some situations acceptable, but eccentric and not neutral spelling of this word. It accents the fact that "ea" here are two separate vowels rather than a diphthong: that is the function of the trema (the two dots).
In practice it's not used much, because it's not really necessary: it does not distinguish it from any other word, and trema is not easy to find on English keyboards: so there is no need.