Bond and Oppell address your question points one, two and three empirically by looking for what they call "unbalanced bifurcations" as a sign of adaptive radiations in a well-resolved phylogenetic tree (spiders), and find that the number of unbalanced bifurcations does indeed exceed what they would expect to occur by chance.
As you mention and as I suggested in the comment, there are some serious problems that arise in trying to answer your question. Many of these are well presented in Cracraft, including assuming that higher taxa are comparable with one another, making arbitrary choices about what rank of taxa to compare, ignoring counterexamples, and qualifying rather than quantifying diversity. I think that Bond and Oppell effectively address most of these problems through rigorous, apriori definitions.
There's a diversity of opinions and evidence from biological systems on your fourth question point. Bond and Oppell connect these significantly unbalanced bifurcations to key traits (like orb-weaving) that open up new adaptive zones. I suppose therefore they might say that there's a fundamental factor (a key trait, and therefore open adaptive zones) driving diversification, and therefore might not agree with you that diversification would necessarily beget more diversification (at least, not in the absence of open adaptive zones). For instance, in Hawaiian tetragnathid spiders, ecomorph niches on an island are filled either through immigration from another island or through adaptive diversification (Gillespie). No ecomorph is represented by two sympatric species, so a niche filled by immigration will presumably not spur adaptive radiation. However, species radiations in Andean lupines (Hughes and Eastwood) and plethodontid salamanders (Kozak et al) seem not to have been accompanied by any key traits or particular ecological opportunity, and could theoretically be driven by the self-reinforcing mechanism you propose. As a related point, I suppose more diverse lineages could be less likely to go extinct; which could support diverse lineages becoming more diverse.
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