Together Forever

• WWizards of the Coast2018-06-08
  Official Ruling
  Support can target a creature another player controls.
• WWizards of the Coast2018-06-08
  Official Ruling
  You can't put more than one +1/+1 counter on any one target using the support action.
• WWizards of the Coast2018-06-08
  Official Ruling
  Together Forever's activated ability checks whether the target creature has a counter on it as the ability is activated and as the ability resolves. If the creature loses its counters later in the turn, the delayed triggered ability will still return it to its owner's hand when it dies.
• WWizards of the Coast2018-06-08
  Official Ruling
  If a creature without counters receives enough -1/-1 counters to reduce its toughness to 0 or less, Together Forever's ability can't be activated before state-based actions put that creature into its owner's graveyard.
• WWizards of the Coast2018-06-08
  Official Ruling
  If a spell with support has other abilities that target creatures, those abilities and the support ability can target the same creature.
• WWizards of the Coast2018-06-08
  Official Ruling
  If some, but not all, targets for a spell become illegal, the remaining targets are affected as appropriate. If all of a spell's targets become illegal, that spell doesn't resolve.

Rulings via Scryfall