Yes. Despite what little league rules are due to safety precautions, there are times where it's faster to slide into first in professional baseball.
Yes
Yes
The answer is no. All little league runners are prohibited from sliding into home head first.
yes
No, in Major League Baseball a base runner can slide into any base in any direction.
He can but it's not wise.
varies league to league. in most baseball leagues, it is probably ALLOWED, but unnecessary (and dangerous!)
Whenever there is a play the runner has to slide. there is no mandatory slide rule the runner just has to avoid contact
No, it's legal to go back head first.
only in little league
Don't know if it's illegal in Little League to slide into first base, but it certainly isn't necessary. The reason for sliding into a base is to try to keep the baseman from tagging the runner. Runners going to 2nd, 3rd, or homeplate must be tagged out unless it is a forced run, in which case the baseman just tags the base. Running to first base does not require a runner to slide since the runner doesn't have to be tagged at first. All the baseman has to do is tag the base.