There is an easy solution if they get to the point of wanting those things, which is to buy the game. It is a free trial, and one of the most generous a game could provide, giving them enough content that many trial players will take years to get through it all.

Being in an FC allows them to do submarines for RMT purposes.
Being able to trade, even with friends, allows them to trade gil for RMT purposes.

The being able to reply bit is probably alright, but the chat restrictions in general are to prevent RMT spam unless they pay SE money.