I'm not that clear with the first question. The traffic from your inside network will take an interface to exit depending on a lot of checks like NAT, route lookup,etc. So, if the route that you've specified for traffic going out goes like "route outside 0 0 ", then this route will be taken for all traffic destined to a network on the outside of the firewall, unless you have a more specific route(i.e. with more specific destination IP addresses/networks).
When the reply comes back, the upstream router will send it to the ASA as long as it proxy ARPs for the destination IP address in the reply packet.
For the second question, you can have the ASA be configured for DHCP relay. Here's a link: