It would be easy enough to verify though: just make a human with 18 str and dual class them right at the start of the game.
Dualclass requires to be level 2.
Bookwyrm627 suggested this back in 2019. I suspect OP has run the test by now, or decided not to bother. Regardless, the suggestion could be read as dual classing "as soon as possible" (i.e. at level 2), or to be that it be tested in Baldur's Gate 2, where the starting level would be high enough. Though OP specified explicitly BG1 for the question, the mechanics ought to be the same, so a basic test in BG2 should be valid for BG1.
Yes the tome increases it to Strength 19.
Bookwyrm627 confirmed this back in 2019, too.
For classes that dont get percentile strength it doesnt matter. You only get one attack per round.
All classes benefit from the improved THAC0 and increased damage from higher strength. Higher attacks per round provides a bigger benefit from the bonus strength, but even non-fighters hit harder with the damage bonus from strength.