At your questions:
1. Speed is just word, we are ninja after all so saying a speed increase would be for relative effect and gaining distance. In essence you're now a snake and snakes move fast, being a ninja snake i'd say you move as fast as an official sage maybe a bit faster. Not quite lee without his weights but close to it. People can see your movements but are hard pressed to react to it. Does that make sense?
2. If you're hit with chidori, you'd be shocked and paralysed like any other ninja. You're immune to damage that would destroy your body for the most part, but not immune to being stunned. Same as being put in a genjutsu, which you'd be vulnerable to as your body is so tough in this state, you'd need a way to counter gen, a huge weakness which is why i think Sasuke beat Orochimaru, using his eyes that way. Now as for fire, it would depend on the size and power. If someone causes an F rank large blast of fire to engulf your entire body, which says it burns anything it touches, it would wipe out your body before you could regenerate. Also the fire would hit your head which would destroy your head the same as having it cut from your body in a sense. If it was a fire stream that hit your chest, it would burn through your chest then you'd reform with smaller snakes.
Now, the biggest thing about this is not being able to use handseals and using jutsu that can me used from your mouth, this will put you at a huge disadvantage and you'd need to be ready for this. Maybe you could make a clone to cover your back as once you're in this form you're all in betting wise. As for when i see it, my first idea was to trap you, keep you away from me, for example use the swamp, stuck in mud, your speed is useless and you can't release the smaller snakes to attack. So beware when using it.