Insulinde I doubt I'd ever get the mercenary flotilla.
Well, Flotilla has some merits if you somehow take it at the very first opportunity, at Ardford summit. For example, by skipping on Ardford Supplier and Deeds of Vinario and taking Flotilla while at the same time reducing Ardford Bank cost to 300k thus making Flotilla's cost 150k effectively. This give you better returns than skipping Ardford Supplier, if you plan on taking Tak'kan Trade, but will make your life harder in the interim (funds spent on reducing chaos after Battle of Yhilin, possibly less recruits for Forging Ties quest, you will be tight on Tak'kan score initially). But it still a possibility.
Insulinde I've also not funded Erosia's smiths
The same. Although It's possible that reforging all Incubus King's gear would require all smiths in the world. Or reforging it better with them, than without.
Insulinde On the other hand, I don't see imp offices as a wasted investment. It's not ideal to pick it up so early, but I'd want it eventually, as helping orcs self-actualise (it eventually improves a hidden stat) seems like a worthwhile outcome in itself. For a similar reason, I haven't focused on attractiveness at the second gathering, even though that looked like the most convenient way to get good immediate results; I don't want to shape the development of a species around the preferences of an empire which treats them as chattel.
Well, from the roleplaying point of view Orc Pools would be unquestionably best investments as it supposed to improve all orcs and in all aspects, not just imps (like offices) or "thinky orcs" (like Hall of Mental Strength). As for attractiveness, again from roleplaying point of view it still make sense. Maybe, not as the best possible improvments for orcs, but as the most pragmatic way to improve overall situation in the world from Simon point of view. He has to consider everyone, not just orcs. And it's not like boost to attractivness is a short end of the stick to them, it is an improvment.
Insulinde At first, I thought buying the Stineford bank before meeting Megail and still getting a return could sort of make fictional sense, if you imagine that it's a sort of shorthand for Simon just having someone drop by after the postwar investment phase... but on consideration I think it truly is an exploit, so would personally avoid it just for that reason.
I rather imagined it as buying a way to bank's board of directors, after which Megail forcefully inserted herself as the beneficiary for annual returns regardless of being very recent addition to core lenders.
But if you want there is an alternate way to invest, that I took initially (still skipping Tak'kan Trade though). Instead of early Stineford Bank [400k], Mercenary office [100k] and Denmiel Mushrooms [75k], I tried Gasm Falls Trade [225k] and Theltiar Rentals [350k]. Most other investments went about the same as I posted above with some reshuffling (like taking Band Tour only in second phase, since it's not needed with Rentals), but by the end of chapter 4 I had exactly the same investments and only 77k Pron less than with early Stineford Bank.