Log: dhcp, warning Detected conflict by ICMP response for 10.0.70.72
This indicates an IP address conflict. How things work: DHCP client requests a DHCP lease. DHCP server decides upon IP address to lease and (optionally) checks if the IP address is in use (it may use ICMP echo request to do it). If check is fine (no host returns ICMP echo reply), then DHCP server offers IP address to client. Client again performs check if IP address offered is in use. And only if that check is fine it starts to use the assigned address (thus finishing DHCP handshake).
The above quoted log might indicate that server detected that it selected (and not yet assigned) address which is already in use. This is not a problem per-se, DHCP server in this case selects another IP address from pool (if a free address is available) ... but it does indicate there's a problem in your network. Either it might be due to DHCP server loosing its state (restarted before writing out lease information) or due to rogue device (uses IP address from pool without acquiring it from DHCP server) or due to another (rogue?) DHCP server present in network ... or due to buggy client device (if it tries to renew the lease but fails to indicate this in DCHPREQUEST message and then replies to server's checks ... or if it tries to re-use previously used address (lease expired in the mean time) and again answers to server's checks).
BTW, I doubt multicast helper can really help to make DHCP exchange more reliable. Multicast helper acts on packets sent to broadcast MAC address ... DHCP packets are exchanged between unicast MAC entities (client with known MAC address, but without valid IP address, and DHCP server ... only initial DHCP DISCOVER packet, sent from DHCP client, has broadcast MAC address set as destination, but that packet is not handled by multicast helper as well because most of time DHCP server is wired and multicast helper only works when AP is transmitting broadcast packets over wireless, not when it's receiving them).