You better use protection because you probably could become pregnant any time you have sex without it. You might not be and you might be.
it starts almost instantly after intercourse is over
If you've gotten your period afterwards, it's unlikely that you're pregnant
With or without protection you can still become pregnant.
Yes, ovulation occurs half way between periods.
Depends on when you last had intercourse. Semen can linger and be viable a few days after intercourse, and rhythm methods are notoriously unreliable. But pregnancy w/o sex doesn't happen among humans.
You may still have a false negative but you are plenty of time after intercourse and you had intercourse at the right time to become pregnant.
Once you become pregnant your periods will stop until after you have given birth.
You can
You can not know that you are pregnant the day after intercourse. You have to wait till you miss your period. There are few sensitive, but expensive tests, those can tell you before the periods.
Do the intercourse in the safe period. It normally starts from 5 days before the cycle and 10 days after the menstrual cycle.
absolutely
No, not at all