1-there's a coolant level sensor underneath that tank, it's a quarter turn
screw/unscrew type, but the bad news for you is that sensor is inside a dry
compartment of the tank....that means your tank is probably scrap due to an
internal crack in the tank. To remove it, your right, you have to lift up by
unscrewing the bleeding screw and bend the little tab to remove the holding
device than pushing the tank a little back you will have to remove the clamp
holding the overflow hose. After remove the bottom hose and pull the tank
away.

2-There's plenty of ground points. The most important to check are the one
on the inside front right rail frame    (passenger side u.s.model) leading
to the engine -- toward the tip of both front fender there is kind of "bus
bar" holdind a bunch of  grounds for light, abs/asc unit, sensor of all
kind, etc. On the left side it's pretty obvious, on the right side remove
the air filter box, it's right there, a flat and black thinny plastic with a
triangular head, where a bunch of brown wires goes inside. If I remember
well there're 2 of them both side, each close to the other one. --another
good place to check is under the carpet of both front seat and yes  you have
to remove seats, side footwell trims, you should be ok without removing the
gas pedal. On the left side there is a bunch of "bus bar" to check for
corrosion (lines or negatives). On the right side you'll find a plastic
cover and underneath there's a junction box and something like 8 or 10
inline fuses a couple inches ahead there's a negatif "bus bar"bolted in the
middle of the floor.-- In the trunk check  the battery negative cable at
both end.-- Those are the proniest one for trouble if water has been true.

3- Sorry I don't know!

good luck!