puck as repeater in order to scan a remote puck (out of range from the master)

Yes, absolutely - all you have to do is mash the two together for a 'middle' repeater:

function setFound(found) {
  NRF.setAdvertising({},{manufacturer:0x59­0,manufacturerData:[found ? 1 : 0]});
}

NRF.requestDevice({ timeout:1000,  filters: [{ name: 'Puck.js repeater' }] }).then(function(device) {
  // repeater found
  setFound(device.manufacturerData[0]);
}).catch(function(e) {
  // repeater not found 
  setFound(false);
});

It's worth noting that you could modify this pretty easily to send the status of more than one device (or even some data from another Espruino) by just putting multiple elements in the manufacturerData array.

is MDBT42Q with Espruino JavaScript interpreter pre-installed, able to be a repeater or an endpoint ?

It can do anything the Puck does, so yes - it can be either.