1.- Before auctioning the private companies, watching the map and to establish the zones of easier development, thinking about putting new stations and hotels.
In 18FL this is very much the case, iirc. In other games, there's privates that are simply way better than other ones, and then the private auction becomes an exercise in driving the price of those privates up to an 'acceptable' level.
it's also important not to get fixated... yes, you can drive the auction off predetermined strategy, but the opposite is more important: to tailor your strategy to which privates you have.
4.- To decide if to invest in a single corporation, the one that the player administers, or to diversify.
I believe 18FL is designed so that everyone can open in at the start. If you can open at the start, doing so is almost always a good idea (there are limited circumstances when it isn't). It's also much more interesting: if you aren't running a company this operating rounds get a bit dull.