How would svellsonger work with Riven

Yes — Svellsongur copies Riven, Shattered's printed text, so she gets a second instance of "When I attack, choose an enemy unit here. Deal 2 to it for each Equipment attached to me."

The copied ability is appended to Riven's rules text, so both instances trigger on the same attack and go on the Chain as separate triggers, resolving one at a time. Each instance deals 2 per Equipment attached to her, and Svellsongur itself counts as attached Equipment — so with just Svellsongur on her, that's 4 total damage (2 + 2), split between two independently chosen enemy units at her battlefield if you want. Two Svellsongurs would give three instances (they only copy printed text, so copies don't compound — FAQ #1141).

One thing that doesn't work: the copied Weaponmaster does nothing. Svellsongur is already on the board when it attaches, so there's no "when you play me" moment for that copied text to trigger off (same reasoning as FAQ #9497 with Ivern). Applies to both Riven printings, since the attack trigger is identical on each.