ksllaw, your questions are answered throughout this thread, but I'll try to answer quickly and get you pointed in the right direction.
ksllaw wrote:a.) How are JAG salaries generally?
It's best to look at the compensation as a whole rather than just salary. You will generally come in as an O-2 and move up to an O-3 relatively soon (after 6 months in the Air Force, but I think it's longer in the Navy, for example). You can look these up on military pay charts online (O-2 is around $40k per year, O-3 is around $45k). You can also check out the charts to see how the pay advances.
You also get BAH and BAS, both of which are not counted as income. This can range from $1,500-$2,000 per month for a lot of markets, depending on location, rank, and family size. So this can be an extra $18k-24k per year, tax free. On top of this, medical is covered, which will save several thousand per year. Some of the branches offer student loan repayment plans worth up to $65,000 during your first 4 years. If you stay in for 20 years, retirement is also available.
As an example, I, as a married guy in Atlanta with no children, would have to make around $75-80k in the private sector here in Atlanta to match my take-home pay as an O-2, and that doesn't factor in the possible $65k in SLRP funds or the value of retirement if I were to stay in that long.
b.) Is it an LRAP eligible position?
I am not that familiar with LRAP programs, but my recollection is that it is for most. Certainly check with your school's program to be sure.
There are other alternatives, such as the $65k in SLRP funds for the Army and Air Force, and IBR & PSLF are available for all branches. Note that your income IBR is based off of would not include the BAH & BAS, making your IBR payment that of someone making $40k a year (to start) instead of that of someone making $70-80k.
c.) Is it at all possible to migrate into JAG later in one's career (post biglaw, e.g.)? It seems everyone is saying the best option is to go directly from out of law school?
Yes, I don't know if this is a terribly common way, but I have met many people who had pre-JAG legal careers, even ex-biglaw employees. One concern is age - the max at the time of commissioning is 35 for the Air Force and 42 for the Navy and Army. Another concern is explaining why you would want to make the switch (but explaining "why JAG?" is a hurdle for about everyone). A final concern is that JAG is often looking for people with trial skills (some branches more so than others), so 4 years of never seeing a client or a courtroom while working for a large law firm wouldn't necessarily be a bonus. I have probably heard of more ex-trial lawyers and people who have worked for PD/DA offices than anything. My only suggestion is that if you want to do JAG, go ahead and aim for it as soon as you can, but that doesn't mean there aren't other ways.
d.) How difficult would you rate getting into JAG compared to getting into biglaw, federal clerkships, and top public interest work? Is it generally easier, harder, or about the same?
The recent selection rates seem to still be less than 10%, and often significantly less. The important thing is that the selection process is different. For most legal jobs, you can calculate your odds of getting the job mostly from knowing your school and class rank. Other factors are fairly consistent for the prestigious jobs too. But the military is looking for someone who can be a successful attorney and who can also be a successful officer - two roles requiring different traits.
JAG officers come from every law school in the nation, and there seems to be little correlation between US News rank and a student's chances at getting into a JAG corps. This has been covered in this thread, so start making your way through it. But the main point is that the hiring focus is different, so having a legitimate shot at being a SCOTUS clerk will not alone get you selected.