Yes, the remote search extracts the corresponding Index Entry Details from the RWI and they become part of your own RWI - so they are used for your search results and stored in your own index.
RWI merges are done constantly in a similar fashion as lucene does that. Two files are merged into one new. There is a threshold when this has to be done but I can’t remember what the exact rules are.
Yes, always - if they are generated at all (there must be a flag somewhere). Indexing in P2P mode should always cause the writing of two indexes, Solr and RWIs. RWIs dissolve during index transmission, Solr indexes stay.
No the entries are selected by DHT targets. Age is not relevant. As far as I remember the DHT target (another peer) is selected randomly and then index entries are selected for transfer. The rule is, that exactly those index entries are selected for transfer which I would expect to be at the peer when I search using the same DHT rule.