Your understanding of the Structural Engineer is correct.
Penny Pincher can use multiple sideways-buildings towards a single purchase.
Drover actually changes the market marker.
The Alchemist spends 1 good. Then, he converts 2 goods (of a single type) to 2 goods (of a different single type).
Thanks! I misread the Alchemist. That's a better deal, spending 3 for 2.