Dear Isaac,
as a reply of your first question that how an outbound roamer will select correct VPLMN when there are many choices.
The answer is:
When subscriber is allowed for International Roaming, the GT (Global Title) of VPLMN MSC is added in subscriber’s HLR of HPLMN in “IMSI->GT translation” section.
On very first LU (Location Update), the request is forwarded by VPLMN network to HPLMN’S HLR and only correct MSC is authenticated whose GT is present in HLR.
It will resolve your query hopefully!