Assuming you can't get a two-flight/p.s. all-UA itinerary via the waitlist, I'd stick with your first plan. Why add all that extra flying and increased risk of problems? Is sitting in a slightly better p.s. seat worth sitting in planes for an extra 2 hours total and having to transfer from T1 to T7 at LAX?
Plus, the US terminal in PHX now has much better food/drink options. Three hours there isn't all that bad.