GuywithaSword, welcome to the forums. Please check out the
forum rules while you're here. We're in the US, which means we have to be especially careful about profanity to keep complaints off our Host--so note Rule #3, and don't use vulgar acronyms like you did, above. I removed it. I'm perfectly capable of cursing a blue streak in reallife, but I'm more interested up here in keeping this six-year-old forum going.
On your first question: are you offered any other dialogue options while speaking with Magnus? If not, your only recourse if you want Magnus back may be to kill the Schuylers, which is probably a good idea in any case.