Is it really so difficult to get the visa
It's not particularly difficult, but it is very involved.
First, you've got to make an appointment at the embassy/consulate. Depending on the type of visa and how busy they are, they may take as much as TWO MONTHS. Admittedly, that is rare and you'll normally find you'll be able to make an appointment within two weeks or so.
But at the very least the wait time is considered significant in some instances, so they've created a whole website to find out the current wait time.
Visa Wait Times - for Interview Appointments and Processing
Then you've got to assemble the paperwork. In addition to the usual form-filling, this also means that you've got to have a photograph taken in a weird US format. Most shops over here are not setup for that format, as Euro-style passports require a different size. So you might have to hunt around for a shop that can do them properly. (Tip: There's one very close to the US consulate in Amsterdam, that gets a lot of business from folks who show up at the consulate with the wrong size photo, and are refused entry into the consulate straight away.)
Then you've got to go to the designated embassy/consulate, of which there may only be one in your country. For some people, that may mean a journey of several hundreds of kilometers. And the embassy visit itself may take as much as three hours, most of the time spent waiting.
And finally you've got to wait a few days before the passport is mailed back to you.
It is not exactly a difficult process but it is involved and from start to finish takes a relatively long time. (Unless they've changed it greatly since I got my M-1 visa five years ago.)
When this was clarified it seems that it was permitted and was described as an "excursion."
I found this, which may be helpful:
5. If I were to go to the U.S. for say, two months, then to, for example Mexico for a month, and then back to the U.S. for another month or longer (total duration of stay in the U.S. at least 90 days), is this possible without a visa? Or does the period of 90 days start again when I arrive in the U.S. for the second time and can I travel for a total of 180 days?
A: The 90 day period starts with your initial admission into the United States. Within these 90 days you can visit Mexico, Bermuda, the islands in the Caribean or Canada and return to the U.S. Under no circumstances can your total stay exceed 90 days without violating the Visa Waiver Program conditions. So if the total trip, including the time spent in the United States, Canada, Mexico, Bermuda, or the islands in the Caribbean will exceed 90 days, you will require a visa for the U.S. If your ticket terminates in Mexico, Canada, Bermuda or the islands of the Caribbean, you must be a legal resident of the area in order to qualify for visa-free travel.
(
Consulate General of the United States Amsterdam, The Netherlands - Questions most heard by travel agents)
It doesn't say anything specific about an excursion by private aircraft, but it might help supporting your case anyway.