short answer yes enzymes convert starch to sugars and yes preboil of oats gelatinizes it and yes oats provide mouthfeel
The first step in converting grain starch to sugar is gelatinating the starches. Think of it as allowing the starches to dissolve in water. Once it is dissolved in water, then the enzymes in the barley malt react with the starch to break it down into smaller molecules. If its not dissolved, it can not react. Starch is a bunch of sugar molecules (monomers) bonded together to make a very long chained molecule (polymer aka lotsa mers ). But the enzymes can not get at the starches to break them down until the starch is gelatinized. Barley and a lot of other malts gelatinize at a lower temperature than non malted oats. That's why Palmer recommends hydration of non quick oats before mashing. Quick oats are already gelatinized that is why they good so quick. Now to be fair, I have seen a range of temperatures and Scott Janish says gelatinization finishes up around 155F and definitely by 162F. So as Janish puts it as long as you are on the hot side of mashing temp, you may not need to preboil non quick oats. As Dr Fauchi might say "out of an abundance of caution, preboil your oats"
Sorry for getting all techy....at least I did not talk about 1-4 alpha vs 1-4 beta glyosidic linkages.