USERRA is hard to read and understand properly without being intimately familiar with how guard/reserves fund orders, and the underlying federal law. The same guy or gal doing the same job at the same place may have different levels of protection depending on the underlying funding. Suffice to say you would be 100% legally protected in this scenario.

I will say that within the guard/reserve/airline world it is considered to be somewhat bad form to get hired at an airline and then immediately bail for extended active duty. The reason is that your seniority and longevity (ie pay scale) accrues while you're away, so a new hire would basically skip the first year of rock-bottom reserve and low pay and return at a higher pay and relative seniority as though he never left). Note that your probation clock (usually one year) normally only runs while you're actually at work (they want to observe you in action, fair enough).

This is obviously tempting, but if everybody did it, guard/reserves would get a bad name in the airline industry. Most of us don't give a whit about the airline management, but collectively we don't want a bad reputation with hiring boards, or our fellow pilots who have to cover for us when we're gone.

But with that said, it's legal and some people do it...a few actually seek out active-duty orders and are gaming the system, others just get an involuntary mobilization at a bad time and have no real choice. In your case you would have a very good excuse...you absolutely DO NOT want to delay your civilian career progression just because you might (or might not) go on mil leave. Also you don't want to miss an opportunity to get a military flying slot. If you can get hired by a major, do it. If you then need to go on mil leave, do it.