Re: 'must' for probability
The use of 'must' to indicate logical certainty is not particularly informal.
It is not generally used if there is a possibility of ambiguity. When it is clear that the present moment is being spoken of, then there is no problem; 'must's uggesting obligation almost always applies to something that must be done after the moment of speaking.
'He must return, he's forgotten his umbrella.' (not for logical certainty)
'He must be very wet; he's forgotten his umrella.' (fine for logical certainty)
'They must win tomorrow, or I'll lose my bet.' (not for logical certainty)
'They must win tomorrow after all the money they've spent on new players'. (possibly ambiguous)
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