1) Writing a letter to get combat related issues for potentially unfit conditions is not a bad tactic.
You can appeal combat related problems you don't agree with for unfit conditions. You don't know what will be unfit, so for now, it potentially makes sense to send info about all conditions. Its not a bad thing to wait to see what they say and then disagree after you get the 199. If they find the back fit, combat related for that won't matter at this stage and the PEB won't say yes or no.
If combat related or not is a determination made by the PEB that will effect you in any significant way is another question entirely. It does not always present a significant issue. I would be surprised if can't rule out the possibility is clear enough for the PEB to agree that it is combat related.
2) The PT test is a factor in determining if a condition makes you unfit, but in reality, your job isn't doing a PT test every 6 months. If you can present evidence that the condition prevents accomplishment of your duties, that's what they need to determine unfit. Your story presented sounds like your knee sucks, which is fine for VA compensation, but not enough to say you need to be kicked out because of your knee. You'll want to get specific into how you can't do things, i.e. your job requires you to work pedals in the jet, which you just can't do anymore. Similar for the back, i.e. you can't sit in the seat for a flight. /shrug
3) Pretty much. The medical people don't think there's any reason for them to kick you out for the other 3 conditions. If you think they should, its a huge red flag with bright flashing lights that you should give them more evidence.
4) The PEB does not determine the rating in the IDES process.
The VA determines ratings, and they primarily utilize the C&P results. It is unusual for the NARSUM to speak about criteria important for determining a rating. Technically they utilize your entire medical records, but the C&P is an exam specifically for finding out all the info a rater needs and is heavily relied upon. You can submit additional evidence to the VA that they can consider as well. Generally by uploading to ebenefits, but the MSC should be able to send it up.
Its fairly easy to determine if information in the C&P will effect a rating by looking at the rating criteria in the VASRD. Its not always smart to haggle over minor wording use without understanding what is important. Example: C&P says you get 2 prostrating migraines a month, the NARSUM says you get 2 migraines a week. Instinct says 2+ migraines a week is worse and the NARSUM is better. Instinct is wrong. The fact the C&P used to word prostrating means you'd qualify for the max 50% rating, but the NARSUM is saying you'd get 0% rating because none of your very frequent migraines are serious enough.
Its important to note the different evidence and who needs what. PEB needs evidence showing you can't do your military job due to a condition. VA needs evidence showing how severe your condition is and how it interferes with social and occupational tasks.
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For the good Charlie: The point was the VA doesn't get to decide only 1 condition is unfitting.
It seems clear there is some level of confusion about what is actually preventing him from performing his job. Tdyslider seems to believe there are four conditions that are below retention standards. Its a bit confusing on what the profile / PCM actually said. The NARSUM seems fairly clear, the diabetes is severe enough to need you to be kicked out. It sounds like tdyslider believes his PCM said his knees, his back and some other condition was also severe enough to be kicked out. Its not clear why the PCM believes that or why the MEB disagreed. Its the why and the how of that disconnect that is pretty important to resolve at this stage.