I'm going to respectfully disagree on point 4, I'm afraid :P
I'd observed that some things are disproportionately effective vs unfinished buildings (most notably archers) and siege is unexpectedly bad. This inspired me to work out the specifics, and it's actually a bit surprising. Normally I'd script a lot of the hard work but I'm not really well enough to do that right now, so I opted for more simple reasoning. If I've missed anything blindingly obvious in here, please let me know!
Scarab (6h, 12c, x5 vs building) does 117 per hit on a house (30% hack, 5% crush)
From this we can work out the anim length modifier: expected dps is 6*5*0.7 + 12*5*0.95 = 78
Therefore scarab's attack interval is 1.5s.
On an unfinished house, scarab instead does 48-49 per hit!
Too many variables: set scarab crush attack to 0. Scarab now does the expected 31.5 per hit (6*5*0.7). Can't remove house hack armour or scarab attack bonus no longer applies, thanks AoM!
Now it does ~16 per hit on the house instead. So what's going on with this? Is it affected by attack bonus or building armour?
Increased house hack armour to 60%. Now does 12 per hit (6*5*0.4) on finished, and 16 on unfinished.
Increased house hack armour to 80%, and it still does 16 on the unfinished -> building armour does nothing here.
If the scarab does 6 crush or pierce instead of 6 hack, it still does 16 (well, somewhere slightly over 16 per hit, but regardless it's close).
If the scarab has its attack bonus vs buildings removed, it still does ~16.
Now the question is where that 16 per hit actually comes from. Using the KB unit info display I can see the actual damage per hit is 16.2.
That would indicate some damage multiplier of 16.2/(6*1.5) = 1.8x
As a quick test, I thought I'd look at something I already know the attack interval of from previous experiments: the white tiger, ~1.83s (see https://steamcommunity.com/sharedfiles/filedetails/?id=859012992)
Based on this (it has 20h attack) it should do 20*1.83*1.8 = ~65.8 damage per hit on an unfinished house, which is what I observed when trying this.
To be sure it's not something specific to houses, I tried on a temple too and got the same result.
TL;DR: Units attacking unbuilt buildings ignore any existing damage multipliers and the building's armour, and instead do 1.8x their base damage.
So yes, the scarab does significantly less damage to unfinished buildings because its 5x multiplier is voided and its crush damage doesn't have the usual benefit of being more effective.