Good analysis! I tend to agree that the theory (hope on the Dem side) that undecideds will break towards Dems (on the grounds that Republicans are the incumbents) is probably exaggerated. That may happen, but it may also not happen. As you outline well, there are examples in recent history of both of those things happening.
Relative to the presidential party, the polls have broken in different directions in different years. So I really don't see too clear of a correlation between a party having the presidency and overperforming the GCB. On the other hand, I do see a pattern of the majority House party generally doing better in congressional elections than predicted by the polls, regardless of who's president.
Like I mentioned, that pattern has held true in 7 of 8 congressional elections since 2002 (including presidential years), and in 4 of the 4 midterms since that time.
Yes, this tends to happen because of fundraising. Incumbents in the house tend to have more cash on hand and will generally over perform expectations. However, if you look at the fundraising this cycle for house candidates that incumbent advantage has disappeared.
