1) Average American League ERAs tend to run about 4.50 or so across worlds. Rivers is a very good pitcher-- but his only OK VsR and only fair GB/FB stop him from being great. In your park and world his ERA will run mid- to high- 3's, just a little better than his current career ERA. He's still really valuable given how many innings he can pitch at that level of effectiveness. I'd use him essentially exactly as pjfoster says, PC 25/30.
2) I only use a closer when a guy BOTH (a) is an excellent reliever and (b) has low enough ST/DUR that I only think I can get 60 IP out of him. On your team I
might close with
Hamlet Hunt-- but I might go with no closer.