Beta-amylase is the enzyme responsible for converting starches and long-chain dextrins to fermentable sugars (primarily maltose). It's optimum temperature range is ~126-149°F. Once you get above 154°F, you have little beta-amylase and mostly alpha-amylase activity, which converts starches to dextrins (mostly nonfermentable sugars).
With your mash schedule, you essentially skipped over the temperature range for beta-amylase. However, these temperatures aren't black and white for enzyme activity. It's best to understand your malt and, ideally, to have the malt analysis for your base malt to choose your rest temps.
For instance, many German brewers who use German 2-row pilsener malt, use temperature rests of ~140-142°F and then up to ~158°F. The lower rest facilitates some protein-related enzyme activity (which you typically don't want too much of) and some beta-amylase activity at the same time. The higher rest then adds body by creating dextrins with alpha-amylase and still has a little bit of beta-amylase activity going.
Your first decoction (or any non-mash-out decoction) should rest at ~158°F for about 10 minutes prior to boiling. This allows alpha-amylase to break down starches that beta-amylase can't break down. Then when when you return the decoction to the main mash, the beta-amylase can make use of these and break them down further to fermentable sugars.
It's unlikely that adding yeast will do much. However, if you pitched your starter at high krausen into your dextrinous beer, you might get another point of gravity or so. This is called "krausening". You also might not get anymore attenuation, so it's a decision you'll have to make if it's worth doing.
