Can fresh semen on the front of a small toilet seat cause pregnancy? (If genitals touched seat by accident)

Sperm + egg. Pregnancy is extremely unlikely in the circumstances you describe. It is always a good idea to wipe down a toilet seat before sitting on it. No one would want to use toilet that has semen exposed on it.
Very very unlikely. If you miss your period by 4-5 days, do a home pregnancy test. If the result is negative repeat the test 4-5 days later unless you get a period by then. Use first morning urine and follow the instructions for the test carefully. If you do not wish to be pregnant, use contraception all the time, every time. You may consider implanted contraceptive, or IUD. Practice safe sex if you have sex. Get HPV vaccine.