1. No you don't need to issue a form to a spouse. Just make a note of how you came up with the division and stick it in your tax folder in case someone asks.
2. You can choose to withhold whatever amount makes you comfortable and you can change it as often as you like. Your employer can't do anything but change your withholdings. Some people take less out around Christmas or summer vacation. You are right to keep an eye on your withholdings and make sure you take enough out. Please see another post of mine to help you with determining what is best for you.
3. That is correct. No need to mention it. Here are the rules, Nonresident Alien Spouse | Internal Revenue Service
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