#1 would be fairly easy. Maybe also reduce the castle requirements for each level.
#2 I don't believe is possible, the income increases are hardcoded on the Trading_Post building line.
#3 On the farming, maybe what is needed is a longer line of Improved Farmland buildings with some non-linear bonuses per level. Instead of the vast disparity between provinces in the base farming value, even it out some but start with a higher farming improvement level building. You could cap provinces with a resource that would be required for higher levels of farming.
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