Substrate return line

If you run a closed loop system like canisters you can drill the bottom pane and have locline run up through the substrate. Wont work for sumps because if power went out your whole tank would drain

Water will drain until the siphon breaks. Siphon wont break until water line gets below the return so um ya.... I wouldnt ever try to set the return from a sump anywhere except the surface. If you want to do this do it with canisters or inline filters like oceanclear or lifegard or something like that. Google "closed loop aquarium" for loads of results

Note the return for the sump high in the overflow while these are separate and used to flow more water near the bottom. Not my pic just one from google

Are you referring to under sand/gravel jets? These are usually run on a separate pump rather than hooked up to a canister filter or sump.