A question about personal unions, cores and buildings:
I know with integrating vassals you do not retain the buildings of the provinces and I think it's the same when integrating a junior PU partner through deliberate diplomacy, but how about integration through Monarch death?
ie. Is there any case when the conquest of a province (aside from reconquest) leads to the buildings being preserved?
Any province within your culture group will maintain its buildings if you inherit a PU, provinces outside your culture group become cores but lose the buildings.
To the person with the trade going to Kiev instead of Constantinople... My observation on how trade works is as follows:
If your trade power is transferring power then it helps determine how much value in a node stays in place or gets forwarded. Once that value is determined, the direction is determined by a similar steering fight. If only one merchant is actually directing trade in a specific direction (or all merchants actively steering trade) then 100% of the "pushed" value will go where they want it. If multiple directions are being "pushed then they have a steering competition similar to the stay/push competition. If no one at all is directing trade power in a particular direction then it appears to split evenly. Your trade power is pushing money out of Crimea rather than allowing it to "stay home", the Polish (or whoever it might be) merchant pushing towards Kiev is directing your power in that direction because he has no competition trying to push it elsewhere.
edit: in your case, depending on the numbers involved, you may net more by moving your merchant from Constantinople to Crimea. You can collect in Constantinople without a merchant and the 2 power may be pretty irrelevant there. However, to push trade in the direction you want it to go you have to have a merchant, all the power in the world is useless without direction...