You can find free music on YouTube. Go to your avatar on the upper rt corner, left click, then see "Creator Studio". Left click, then look on the left, find "Create", then left click. Audio library. Free music, ad supported music, and sound effects. You can download the free stuff.( Which I do).
Also, another "cheat" is to find non original music posted on youtube and copy it using a program that will use just the audio( youtube to mp3 converter) and save that music. example: a garage band playing a Jimi Hendrix tune. I believe that is copywrite free.
Nope. That'll still get flagged. Basically because every song has 2 copyrights. 1 for that performance/recording and one for the original composition. (The music and the lyrics). So by using music in that way you'll actually be violating 2 separate people's copyright!
And trust me, youtube is good at recognising covers. I have an acoustic covers channel and if it's on their list, they'll recognise it, fast!