I have Marriott Gold status through existing gold with SPG. Will I rollover nights above my "earned" silver membership qualifications for 2018? If I rollover nights into 2018 I should be able to qualify for Platinum.
No. If you have Gold status, only nights in excess of 50 will roll over. It doesn't matter how you reached Gold (in your case with the SPG match).
Until the programs combine, it's probably best to try to reach Gold/Platinum with either MR or SPG. At least that way you'll get the status match. That doesn't mean avoiding hotels in the other program, though unless you end up staying in excess of 100 nights, focusing the majority of stays/nights with either SPG or MR will provide the most benefit. Once you hit Gold (or Platinum if that's your goal), splitting stays between the programs makes more sense.
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