You can place the sand in zip loc bags, lower them to the bottom and slowly open and pour the contents out. Another way is to use a piece of 2" PVC and a funnel to pour the sand on the bottom where you want it.

Baffles are almost a must if you have an in sump skimmer. Microbubbles will be a major PITA without them. A Mag 7 is a monster of a pump with a 10G sump even with baffles. You want some detention time in the sump to dissipate bubbles, you will be recirculating in a matter of seconds with no bubble relief. I don't think you wil be very happy with the results.