By default, the publish interval is one half the sampling interval for your monitored item - and you specify the sampling interval with each subscription. It is possible to change the publish interval separately from the sampling interval, but there should be some specific reason for it. For start, on a slow connection, I would just make sure that your sampling intervals are not unnecessarily short - and the publish interval will be adjusted with it automatically.

On a slow connection, with the recent versions of QuickOPC, and if the server supports it and your usage case allows it, it is also possible to enable queuing: In this case, the server buffers multiple notifications even for one item internally, and then sends them all at once with once publish. This allows the "publishes" occur with less frequency; no change is lost, but you do not get the new value right when it changes. If this is acceptable in your usage case, you can set the QueueSize in UAMonitoringParameters to a value >= 2, and then also the PublishingInterval in UASubscriptionParameters can be made longer.

I will later reply to other aspects of what you have posted.