Also, it's best to try to have at least one or two cases where you can
watch someone testing your game. Be it a live stream, or someone you know in person. You can see things of great value when you watch people play, especially in terms of menu layouts and map layouts. Do they often go the wrong way somewhere? What is causing this? Do they have trouble navigating the menu? What trends stem from these problems and how can you fix them?
Testers won't often mention these things when they write back to you, but watching them play, you can learn things like "Hey, maybe I need to make the entrance to that building more obvious." or "Maybe I should switch these two things in the menu so it's easier to access that one they ended up needing more."
There's quite a few people who live stream games and give crits on this site, watch for them to mention they are looking for requests and such. Seeing someone play test your game is just super valuable.
About the plagiarism, I'm not referring to the stealing of ideas, but the whole game itself. When you give the game to someone else, there is always the chance, that that person may present it as it's own.
Get a feel for the community and you'll quickly start to see who is trustworthy and reliable. There aren't a lot of bad apples on this site though. A lot of games get posted in the early feedback threads often and I cannot think of a case of something like that happening on here. (I could be wrong?)
Also, the best defense against plagiarism is to share your stuff. The more often your work is associated with your name, the less likely someone can get away with stealing it. Share development posts, share your ideas, screenshots, etc etc. Send the game to a few different testers. Your strongest offense against plagiarism is building a reputation and name and a close knit group of people who will recognize your work and call someone out the second things go sour.