Well, as a Washington resident, I know Washington DOT at this point is much more interested in more Seattle-Portland service and cross state service than increasing Vancouver service past two round trips at this time. Getting some bang for the buck east of the Cascades from passenger rail investments has a significant political dimension.
When they do, they are not afraid of investing in rail infrastructure, even in BC. However, I do not think they'd be interested in funding improvements to the Fraser River Bridge, a major choke point, because most benefits would accrue to CN.
You are correct that CBSA would likely be an impediment to increased service. They did their best to block the second train when it was originally initiated, and it took major lobbying by the BC provincial government and the City of Vancouver in Ottawa to get CBSA to back down.
As the sponsor and funding source of rail service to Vancouver, unless and until Washington DOT gets interested in increasing Vancouver past two trains, nothing will happen. Having gotten the second train restored, their priorities now lie elsewhere.