You only need to attach the EA to multiple pairs if you want to have different settings for each pair.
No, the MagicNumbers are only if you want to manage trades generated by another EA (Trade Robot). If you want to manage manually places trades, you can ignore the MagicNumbers.
I would love to have partial close function. There is a free tool that has TS + BE for 3 positions. It is great, but it also has 100 other features that make it too messy…
I hope you can make something great!!