Sorry don't have the sources in front of me. If someone can write a more detailed answer with sources feel free to delete this one.

The emperor wasn't powerful during or after the Sengoku Jidai. Even after the Meiji Restoration he wasn't powerful either.
The last time the emperor had any actual power was probably the Heian Jidai, ending in 1185 with the Genpei War. However even during Heian and before it, most of the time power was in the hand of the empress' family ruling through regency.

I wouldn't say Bushido was more martial or honor-bound than Chivalry. They were local variations of the same thing: a code for the military ruling class to keep their rule over peasants.

As for seppuku, no one can say for sure. It seem to start out simply as a way to prevent being taken alive and be beheaded like a criminal, but somehow somewhere got turned into the prefered way of suicide, and got used a lot in the Sengoku by the daimyos to weed out people they don't want and at the same time prevent rebellion (kinda). I don't think it was thought of as "honorable" until the Edo, where influx of Confucianism plus government needing something to control samurais who have no wars to fight created the ideology of this honorable suicide.
And of course there's Meiji militarism playing up the idea.