http://img-fotki.yandex.ru/get/9667/20585795.15/0_bae7b_c435ebc7_orig
In left case bottom inserter slightly faster than top. You can check it if you temporary remove bottom inserter.
When inserters unload wagon left inserter 2 times slower than middle inserter.
When inserters load wagon both ...

I did some testing.
I feed output of unloading station to loading station.
And found some strange behavior. Loading inserters work with different speed and faster than unloading. Unloading inserters work with same speed.
Anyway my stations do almost perfect balancing and it good enough for trains ...

Balanced loading station not a problem.
Only problem with unloading station is inserters at center of wagon works much faster then inserters at ends.
Because of this chests at center of wagon receive more items than chests at ends.

Kazuar wrote:An alternative to polling the status of gigabajilions of trees for the need to regenerate, sixty times a second each (and let's be honest, Factorios forests are dense),

Not a problem. Just add tree to collection of damaged trees when tree damaged. And remove when completely healed.

I think it's better just replace block signals with path signals.
All existing railroads will work as is.