1st scenario:
Make another column in Client Table called “Client’s Projects” and give it this formula:
[Project Table].Filter(Client=thisRow)
This will give you a comma separated list of all of a client’s projects.
To take it a step further, you can wrap it in…
BulletedList(...)
…for your 2nd scenario.
And because Coda is great, this will be a list of links to the actual projects/purchase orders; you can hover to see info or click to edit in a popup.