If I understand it correctly, you want the ERS4900 to act as the FA server, and the X435 as FA Proxy, correct?
If yes, the "Configuring Fabric Connect on ERS 4900 and 5900
" guide should help:
1. DHCP is handled by a DHCP server or relay in the respective subnet, this has nothing to do with tagging/untagging the clients or the switch management.
2. Yes, as long as you create the VLAN - i-sid/nsi on both sides, for example manually or using RADIUS, FA will stretch the VLANs over the link between ERS and EXOS.