What sort of venture would you give a DoH retainer that's comparable to the hunting/mining/botany/fishing ventures? Would you have them make crafted items, dependent on having the required materials placed in the retainers inventory? Wouldn't that just add to more market saturation of goods?

Would buy orders share listing with sell orders or would their limits be separate?

I do like buy orders as an option for players but I really don't think it's going to fix any problems with the current market. The saturation and low prices comes from supply compared to demand. To fix saturation, SE would need to put limits on supply or add new uses that create more demand.

Why would anyone sell that belt they got from a quick venture to a NPC for 24 gil when they can sell it to another player for 500 gil? It might seem like a silly low price but it's still more than they'll get from the vendor. SE is highly unlikely to increase the sell to vendor prices to make them a more attractive option because that would feed the RMT farming bots more gil for the items they obtain while leveling.

I don't use the Companion app so I can't add much comment on that but I fail to see how it makes anything more real time than it already is by simply logging into the game. You should have to log into the game to play the game, not do it remotely.