KBG spreads so it may fill in your bare spots. Anything you planted last fall is still getting established itself, so don't expect a whole lot of spreading yet from the new grass. The older grass should spread. Fall is a better time for seeding. Tenacity gives only a few weeks protection against weeds and you are limited on how much you can use in a year. Tenacity at the 4 oz/ac rate is 0.55 teaspoons in a gallon (or more) of water over 1000 square feet (8 oz rate is 1.1 teaspoons over 1000 square feet) . You should use a sprayer with a fan tip nozzle and you should practice (with plain water) to see if you can spray it evenly over a 1000 square foot area. It does give grass seed a head start over weeds in getting established.

Difficult to get kbg going for that long but if you are willing to irrigate, it can be done. I totally agree with Beckyinrichmond about spraying. Takes some practice but you can get pretty good at 1 gal per 1000 sq ft. For tenacity, i like 2 oz per acre but more frequent applicatios rather than a whopping dose up front and then resumed after 4 weeks.