Here is the deal...
If you are referred MEB by the National Guard, you will need a line of duty investigation for each of the conditions that you are referred for. If you do not have one, they will need to start the process.
If you have 25 years in, and have a 20 year letter, you have earned that retirement and will be able to collect it at the age of 60, or sooner if you qualify under the NDAA of 2008.
Depending on the ratings percentage from the NG, you may get a medical retirement (30% or greater rating). Now to play devils advocate, if you have had medical conditions that go all the way back to desert shield/desert storm, why do you not have a VA rating for them?
The reason I am asking this is that the National Guard can easily deny service connection for them and you will have an uphill battle.
Now, let me get to the question that you most likely want answered, if you were given a medical retirement from the National Guard for PTSD, fibromyalgia, IBS and sleep apnea, what would the rating be. In order to determine that you would need to have a C&P evaluation and the rating determine.
For the sake of demonstration, I will make the assumption (loosely) that the NG will give you a medical retirement at 50% and the VA will rate you at 100%
Your medical retirement would be the greater of years of service you have (computed by by each day of active duty you service plus any points from drilling) or you rating percentage multiplied by your average of top three pay. In almost every single circumstance with a reservist, it is the latter of the two.
I will use $4000 a month as a number for you high three. $4000 x 50% is $2000 per month (this would be your medical retirement from the National Guard). At 100% VA C&P you would receive approximately $3000 per month. As you read this you are probably thinking you would get $5000 per month, but that is not the case. Federal law will not allow you to collect both payments at once, so they will calculate what is most advantageous to you which would be the VA C&P payment of $3000 per month tax free.
Now lets get back to your 25 years of service and the 20 year letter if you have it. If you do, then you can collect you non-regular (NG/reserve) retirement at age 60 (or sooner if you qualify for early age drop) and your VA C&P payments concurrently.
I know this may still be confusing so please feel free to ask more questions if you need further clarification.