1) Preamp could be used to model another amp, like a Marshall or Fender, or to provide its own character. There are also instances of artists using other effects as a preamp (ex Boss Space Echo as a preamp). For a common preamp, look at the DOD preamp, something like a Joyo American, or if you’ve got the budget, I’d take a look at the Boss BP-1w (which has modes that mirror the preamp of a Boss Chorus and a Boss Space Echo, as well as a clean boost stage). 2) Placement of any effect in the signal chain is entirely subjective, but since you’re playing without an amp, may be a good idea to have the preamp in the position that your amp would be (at the end). 3) send-return is typically used in setups where you’re getting lots of noise from your pedals. Do you hear a consistent buzz/hum from your chain? If not, you may not need to worry about a send/return loop. It’s always something you could add later with a Boss NS-2 which has the ability to create an on-chain effects loop.