Could/would/should

A: Where’s Julie?
B: She’ll be at home just now. She’s always there in the mornings.

Explanation: We can use ‘will + infinitive’ to make guesses about the present, especially about someone’s typical behaviour.

I got the above from a quiz of a web site on English grammar.
Is the explanation correct? I usually use “could” or “would” or even “should” for making guesses where the probability is the least with “could” and the most with “should”. Am I also correct?
For instance, I would say “I would go” or “I could go” for less certainty instead of “I shall/will go”.

I learnt British English ages ago that we used “shall” for “I” and “we”, “will” for “he” and “she” and “they”. In the meantime we used “will” and “shall” respectively for more certainty, such as “I will” and “they shall” I don’t know if it is still true. Does the American English do the same? I wonder.

The phrase “just now” as I understand from a dictionary can mean “at this moment” or “a little time ago”.
Is “just now” the right choice of words? I usually use “right now” for the latter meaning. Is my way of saying correct?

Since “just now” has two quite different meanings, does which meaning is used depend totally on the context which might not be explicitly stated in the same sentence?