In the comics, Loki always appeared more like an Asgardian than a Frost Giant because he was a runt. In the Marvel Cinematic Universe, there is no clear explanation for why his appearance changes when he is adopted. Most fan theories fall short, and it’s probably a small plot hole that doesn’t really change much.
Arguably, the two shields that Wakanda provided to Captain America in Infinity War were more than proper. He used them to very good effect in the final battle of the movie. Only Thanos with all of the Infinity Stones was able to overcome Captain America with his two shields, and they were clearly well-made.
The dwarves actually did fight against Sauron on more than one occasion. Their exploits were largely excluded from the events of The Lord of the Rings trilogy, but in appendices and other extra sources, Tolkien explained how the dwarves fought. They largely fought in the first war against Sauron and at New Dale.