The OT sheds light on the NT and you can't say you are rightly dividing the Word of Truth if you decide to ignore the older half.
Yes, and the other way around even more strongly. The NT stands quite well by itself, but the OT can be studied to see where the changes were made. Fulfillment of the Mosaic law indicates completion, and this is all explained in the NT. Once you 'get it' you will act correctly without needing all the rules spelled out.