What is the water-resistance / waterproof rating (IPX rating)?

This unit is IP66.

What is the weight of the unit?

This unit weights 4.8 oz including the metal footing.

Will this unit work on the back of a snowmobile Helmet in very cold temperatures and hard bumps.

operating tempreture is 14 to140 F, and storage is -4 to 140 F.

I need to purchase only the magnet bar. After an accident I now need to buy a new helmet, but I need a new magnet bar. Do you sell it separately, and how much is it?

please search for SRHL5 on www.SYKIK.com or see link bellow for a pack of 2 :
https://sykik.com/products/metal-footingfor-srhl5?_pos=1&_sid=791aa45e8&_ss=r

I want to install it by my license plate on my Motorcycle with ABS (need to keep magnets away from ABS breaks). Can I install this without the magnet bar and will it work?

The unit will work without a magnet bar. But the magnet is installed inside the light unit. If you wish, we can have it removed. You will need to let us know when ordering.
If you have a bigger bike, you might want to take a look at " SRML3" 
 https://sykik.com/products/brake-light-for-baggers-and-touring-bikes?_pos=1&_sid=08c36476e&_ss=r

Is it appropriate for escooter use? Also, my helmet is not a motorcycle helmet but does not have any vents or holes on the lower back area.

It will be fine for scooters as well. You can also take a look at SRHL1, a little smaller unit

Can it be set to steady red? Local laws prohibit anything other than red on the back of a moving vehicle.

You can choose the RED color lights. You will have option to have RED in different paterns, such as flashing, sliding...

Is the light visible during the day?

yes, it is. Off course not as bright as night time.