Spirit of Dread’s anti-healing ability isn’t so bad. In Standard, the majority of the time you heal is during the fellowship phase when Spirit can’t even be in play
They only add to the archery total if the fellowship is at FOTR Block site 6, or you use a card like Swan-ship of the Galadhrim to allow them to participate.
Nenya, Ring of Adamant’s text is all effect. That is, you can use its ability even if there aren’t 2 companions to be healed. But, as always, if there are 2 to heal (or just 1), then you must heal...
And to further expand on that example, if Merry’s text did indeed say that, then you couldn’t use his ability unless there are 2 Rohan possessions you can play in your discard pile.