You CAN NOT DOWNLOAD MORE THAN 100, PERIOD.
Deleting them from a dumped save DOES NOT MATTER.
Here's how it works.
Every single copy of the game has a unique ID code.
Nintendo's Wi-Fi service keeps a log of these ID codes.
Lets say, just for example, your legit cart's ID code is 5550022.
You go and download 20 songs, putting you at 80 remaining.
Every time you downloaded one, your cart basically said "This is 5550022, I'm downloading Song A, I have 80 songs remaining to download"
Nintendo's computers check "Yes, 5550022 does indeed have 80 songs remaining according to our database. You can download."
But lets say you got down to 0 left, and deleted to delete some from the dumped save of your legit cart. You now have 20 songs free, according to your song list in the game.
You go on Wi-Fi and your cart says "Hello, this is 5550022, I have 20 songs remaining, I'm gonna go get this song and-"
But Wi-Fi notices something odd! 5550022 actually has ZERO songs remaining! 5550022 tried to cheat the system!
5550022 is now banned from Wi-Fi
You cannot cheat it.
And the reason why you can't download from the dumped ROM is because it has a single ID, and when it was first dumped, a bunch of people tried to get some songs, and the ID quickly hit 100. It is now maxed out and banned from Wi-Fi.