I don't know what to tell you in this case bro. Those dark spots are definitely from an excess of N and that small amount of white around the edge of the one leaf looks a little like lockout but not burn. Burn will normally start at the tip and progress inward and would also be showing on the new growth, which it isn't.
Here is what I am thinking and maybe it will help you make your mind up about how to progress. The older leaves do look dark but it isn't that bad. The new growth actually looks pretty good. You really have two options, you can flush and may be able to get some nutrients out of the soil but I don't think it will help too much because we aren't dealing with chemical nutrients you have added. Those would be easy to flush, these nutrients I believe are more soil based and going to have to break down over time. Or, you could continue to water and hope for the best and not add anything to it. This is the path I would probably take, just keep watering and try to let the plant work through the high nutrient levels because she isn't burning yet. If she does start burning then I would begin flushing immediately but at this point I don't think it helps you much. I don't know much about soil and haven't dealt with MG soil at all so take what I say with a grain of salt. I am just thinking aloud about how I would handle the situation if it were me. It may or may not be the best scenario for you. Best of luck.
As for the ph meter, there are several and it all depends on how much money you want to spend and how reliable you want it to be. To me, Blue Lab makes the best, by far. I own a Hanna 98129 because they are very reliable and about half the price. That being said, I am looking at upgrading to a Blue Lab in the next few weeks because they are a much better meter for another $100. But, the 98129 is an exceptional meter as well for ~$80 and will work just fine. It just needs to be cleaned and calibrated quite often to keep it accurate.
