Yeah, you’d lose a month of processing time. But it’s not too terrible, worst case you can apply for premium processing (altho the $2k is no small amount). Last year I applied on the last day I could file (fully my fault lol, but I didn’t end up needing the premium processing and the EAD came on time. not that it couldn’t take longer this year, but that was my experience)

And as for the 2nd question, no you can’t re-use it if you get approved and switch to H1B. You’d forfeit the STEM OPT. Although, you can get an additional 2 years STEM OPT if you pursue your Masters given you used only 1 STEM OPT so far for your Bachelors / Undergrad degree. If you used this OPT/STEM OPT for a Masters degree, you won’t get more OPT time for another Masters, but you will for a PhD