When Wildclaw Shaman is played, does its 'when you play me' trigger go on the stack after the unit enters the battlefield, and can you use buffs created by other 'when you play' triggers (like Cynthia of Cloudfield) to pay for Shaman's ability?

Ruling: Yes, Wildclaw Shaman's 'when you play me' trigger works like MTG's 'enter the battlefield' triggers, not 'cast' triggers. The unit resolves and enters the battlefield first, then its trigger goes on the chain. You can order simultaneous triggers as you want and spend buffs created by other 'when you play' triggers (like Cynthia) to pay for Shaman's ability.

Sequence:

• Play Wildclaw Shaman
• Shaman resolves and enters the battlefield
• 'When you play' triggers (Shaman's and Cynthia's) go on the chain in the order you choose
• Shaman is now a valid target for reactions while its trigger is on the chain
• You can spend the buff from Cynthia's trigger to pay for Shaman's ability

Nuances:

• Stand United is an action and must be played on an empty chain unless it has been hidden for a turn (then it gains reaction)
• If Stand United is played as a reaction from hidden, Shaman must already be on the battlefield to be a valid target
• Riftbound has almost no 'cast' triggers; the closest equivalent is 'on target' triggers