The classes also get to allocate a bonus, which includes the prime stat for the class, so in fact the race/class combos suggested in the playtest material will give matching non-humans the options of an 18 using the default array, and humans the option of a 19.
I'm not seeing how you get the 19.
Here's my math: 15 (highest choice in array) + 2 (for human) +1 (for class) = 18. Is there a flaw somewhere?
Yeah, and being a Dwarf is the only way to start with immunity to poison. Different races are different.
Your risk of getting poisoned is very campaign specific. In my experience it usually every few sessions. You will use your highest stat every single time you roll an attack.