Learn English – “Meet up” vs. “meet” vs. “hook up”

I don't think formality comes into it, but these two individuals seem to be under the misapprehension that it sounds more formal.

You could argue that, "planning to meet," allows for some flexibility, and the understanding that the plan may have to change, but in reality that would also be the assumption for, "let's meet at three," as if something comes up, plans may need to change.

I would put it down to the facet of human nature that likes a little bit of self importance. "Planning to meet," is just a bit redundant.

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".