1) maximizing chance of having visa approved
Being Americans help; you're allowed into the embassy with the helper. If you're lucky, the consular officer at the window will even allow you attend the 'interview' to help your maid answer questions.
2) understanding the wage requirements -- the salary we provide here is, as typical, far below what the hourly rate would equate to in US minimum wage. Three weeks working for us in the US at minimum wage could equate to 2-3x her salary here in Singapore, would we be expected to actually pay that, and how is that monitored/tracked by US government?
So here is my suggestion for this and the rest of what the govenment asks for:
Help your maid fill out the B1/B2 app. Don't lie. Anything that requires documentation, put in a nice bound plastic folder. Say birth certificate, divorce certs, anything that may be relevant that you would reference in the app. Write a letter explaining the situation. You need to tell them you're planning to take care of the maid, she'd stay with you, and that you'll cover all of her expenses. Now you must demonstrate both 1) your ties to Singapore and 2) your income (warning: if you falsify your tax returns or FACTAs, just stop now). Get proof of your income and such, and put it along into that folder. Make a nice index along with your letter and explain what is on each page. The consular officers *love* when people are organized and have everything they can possibly need. Keeping them happy is your only (long) shot at getting this visa. Anyway, to answer the question your maid won't make enough money and you'll have to hope they'll accept your promise to cover her expenses and accept responsibility for her.
3) I've read conflicting and unclear things about implications for paying US wage tax on our helper's salary for the period we are in the US. Could anyone clarify what is required?
Not sure. You should bring her in as a tourist for holiday, and not as an employee of yours.
4) Timelines to expect to go through the process? I presume we can't buy her plane tickets until an application is approved....
You'll know immediately upon visiting the embassy if she's approved. You basically:
1) Fill out DS-160 online
2) Go pay the fee at StanChart and
3) Go back online and schedule an appointment. Usually you can get a slot in less than a week or two.
4) Go to the embassy at your appointed time. Expect to spend at least three hours there.
That said, I wouldn't give yourself much hope. What Strong Eagle said is likely true. It's all going to be at the whim of the consular officer.