1. That is aggadata and aggadata is not to be taken literally (Rambam Intro. to Perek Chelek). Rav Kook explains in “Orot” that we received the taryag mitzvot because that is our natural lifestyle whereas the natural lifestyle of the other nations is the sheva mitzvot. This could be the meaning of that midrash.
2. It is not clear if Mashiach is before or after techiyat hameitim. It seems from Rambam that it is before. Thus, only those alive then, Jews and non-Jews, will see his coming. In fact, when Rav Arye Levin was asked to curse Nasser ym”s he said that he was not in the business of cursing but would give him a beracha that he should see Mashiach’s coming.