To a modern audience, the main association with this word refers to someone whose features are plain or unattractive. However, in this context the messenger wants to come across as friendly and unthreatening, so the connotation here is that he is plain and simple in an appealing kind of way.
Lady Macduff's son jokingly answers her question with a simile which his mother then continues this idea of her son as a bird with an extended metaphor. His response of "With what I get, I mean; and so do they" means that he will get by however he can. Their witty repartee reveals a fond and loving relationship and makes the end of the scene all the more tragic.
This is an example of verbal irony in which Lady Macduff says one thing but intends to be understood as meaning something that contrasts with what she says. In this case, she tells her son that Macduff is dead even though the boy knows this is not the case.
By "ourselves," Ross means that they do not know one another. Because of Macbeth's spy network throughout the country, it's likely that many innocent and good people have been denounced as traitors, and this would make everyone suspicious of each other. This line helps to reaffirm the tyrannical qualities of Macbeth's leadership.
Ross uses a shortened form of the word "cousin" to fondly address Lady Macduff. While "coz" can maintain its connotations of fondness and endearment without specifically referring to a family member, it is clear that Ross is actually related to Lady Macduff when he calls her cousin later in this speech.
Lady Macduff means that even though she believes Macduff took no direct actions that revealed him as a traitor, the fact that he ran away in fear makes it seem as if he were a traitor to others.