1. If you disable filtering for "synchronous only" messages, then you will also be able to see the backchannel traffic. However, the backchannel traffic shares the same channel ID as the forward message and only differs by the timing, so you'll have to differentiate messages by the order/timing in which they are seen.
2. For the nRF52 series, there will be an upcoming feature to configure the event queue size, but otherwise your only options are to either try and clear events sooner, or to use event filtering to try and reduce the number of events you need to handle.