Learn English – What’s the difference between ‘shatter’ and ‘splinter’

Modest and decent have overlapping semantic ranges. They can mean the same thing, but do not always. To further confuse things, they might get used in the same situation to highlight slightly different aspects of a thing.

Modest can mean behaving in a way that does not draw attention to oneself, of which clothed in such a way that it conforms with sexual norms is a specific case. It can also mean simple because simple things do not draw attention to themselves. From simple, the related meaning of not wealthy or not opulent emerges.

Decent can mean meeting expectations without having the connotation of exceeding them. It can also mean a morally good person, though not necessarily a heroic or saintly person.

Here we have the area where the two meanings overlap: they both imply good in a way that does not attract too much attention. But they are not always interchangeable.

So as to your sentences, I believe the average American reader would consider the sentences about the house to be more or less equivalent. The average American, though, would sense a slight difference in the sentences about the woman, because "decent" is a broader category than "modest". A woman might dress modestly, but lie, cheat, and gossip, none of her acquaintances would call her "decent".

The standard idiom word for word doesn't particularly relate to "translation" contexts at all. It's about accurately copied/reported words, as in the book title...

Elvis: Word for Word
What you'll read are Elvis's words and only Elvis's words. From his first minor radio appearance to the last letter he wrote before his death...

In a translation context, such as this page explaining the French expression...

Appeler un chat un chat
Meaning: to call a spade a spade, to be honest and frank, to tell it like it is
Literal translation: to call a cat a cat

...we're much more likely to describe the "exact" translation as literal, not word-for-word.

There are no dictionary definitions for word by word, because it's not an idiom as such - it's just one possible form of the X by X construction (a succession of consecctive X's)...

Foot by foot, and house by house, the enemy were driven in towards the Plaza.
Day by day he worked on, and day by day the postman delivered to him rejected manuscripts.
Book by book, he decimates the Bible for factual errors and internal contradictions.

It's perfectly possible to translate "word by word" or "word for word". Of those two, the second is about ten times more common, but I see no reason to suppose there could ever be a difference in meaning. Overall though, there are far more written references to "literally translated" and similar variations.