To me both attacks seem to be covered by what is described in the hurricane scenario here.
Here are my considerations about that, which started to become lenghty, sorry:
If there is no willingness co connect back, it depends on the resilience factor of the happ whether data is lost or not. That factor is crucial for your question if the network could heal itself fast enough.
But my first guess is, you need to control an very big portion of the network, to have a chance of causing trouble.
I would say a priori for an entry the probability to be lost is PortionOfTheNetworkLeaving^ResilienceFactor
Say the resilience factor is 10 and the attacker controls half of the network. So for any entry to be lost all 10 members holding the entry must be in control of the attacker, which is .5^10. That means about one in thousand entries is lost. With a 25% attack you break only one in a million entries. Now it depends on the happ how fatal that is…
The greatest damage i can think of that you can deal with this in the example of HoloHost is, that you do a lot of transactions, outgoing from accounts that belong to the attacker, but stay in the network, and receive some return for those transactions. Then you make the exit attack and hope that some of your transactions are lost. However the receiving member of the transaction will still have it. So you try some kind of double spend. You delete the transaction from your chain and spend the money again fast, then the receiving party does not know where to look for for the transaction you deleted.
Since your header is stored in your neighbourhood and the transaction is stored in its neighbourhood, you need the lets call it “p^r-superiority” in both…
And this would be easily preventable, by after such a noteable event of a big portion of the network leaving, waiting a bit for the heal, that the data from the member that received the first transaction spreads through the network before one accepts further transactions.
Long story short, to me it seems like an attack vector to keep in mind, but one that is in most cases unproblematic.