First off, the shrimp are very sensitive to ammonia and based on your results I think you have the answer to the why did they die question...
Regarding the other parameters, you are correct, they are way off from the normal, but I see the place you had your water tested did not do a complete test. What are your calcium and magnesium at? Adding buffer will only affect the alkalinity and will not touch the other parameters needed. Look into Kalkwasser (Mrs Wages Pickeling Lime) or a DIY suppliments like what bulk reef supply sells so you can stay in balance. I am guessing the LFS sold you the buffer to fix your alk parameter...
When a tank starts into a mini-cycle after the initial setup cycle, there is a reason... You need to find this reason, but in the meantime to prevent further loss, you need to control the ammonia levels. You can help make it less toxic by using Prime. Another option is repeated 20% water changes to keep the number down. As the cycle progresses you will be able to lessen the changes or addition of Prime. Prime will not make the ammonia undetectable on tests though, so the water changes AND prime is the best solution.
Regarding the cycle, how much live rock do you have in the tank? Do you run a skimmer or other equipment? We need more info on the life support system to make any further recommendations...
Adding the Phosguard to combat the phosphate levels was a good move. Just don't reduce them too fast or you can shock the inhabitants.
Regarding the high salinity, you can lower salinity as fast as you want down to the target value. Raising salinity is when you need to do it slowly. However I need to ask, how was your water tested? Was it tested by you or the LFS? Was a refractometer or hydrometer used? The reason I ask this is salinity is the bare bones basic value we need to keep in check. You should be doing these tests with a well calibrated seawater refractometer. They are not expensive and will truly save you many headaches in the future.
I do agree there are numerous ways a reef tank can be run. I personally do not do more than monthly water changes (if I remember to do those) on my tank and maintain the parameters through dosing pumps and DIY suppliments. However any time I have ever run into an unknown, whether that be an odd algae growth, coral recession, or fish acting weird, the first thing I do is drag in my water change buckets and mix up some saltwater for a 25% water change... This doesn't happen very often, but it is the easiest way to bring things back into equilibrium in the tank.
I do feel your best course of action is repeated water changes until things look good again. Then figure out your maintenance schedule after and find the tank's sweet spot of equipment, dosing, biological filtration and water changes.