But that's not mistaken. If your steel trades sit untaken that is in fact the fault of your FS. You either need more active or bigger FS members who can handle your trades.
Sure, but it isn't a general problem of same-tier trades.
As we've shown in this and dozens of other threads ross-tier trades are extremely hard to make "fair", so there is often a feeling of resentment on one end of the trade. By simply enforcing a FS rule that each player must balance their own city among tiers you prevent any player from unbalancing another.
I don't get what part is hard to make it fair. Either the trade gets taken and both sides are happy with it or nobody takes the trade and needs adjustment. Maybe other player have other experiences, but I never had problems with cross-tier trades I wanted to make (or taking some, I simply take the ones I like and leave the rest alone). It is just an additional option and can do some balancing.