From what I understand you can add lactose at any point in the process, even at bottling, because it will not ferment. I guess you would have to disolve it in some hot water (boiled to sanitize). That said, I don't know if an amber would benefit from it. Good luck!
Yeah, I hate the stuff. But I don't have a sweet tooth at all, and don't like sweetened things.
If you want more mouthfeel, the thing to add would be maltodextrine. It is a "thickener" and I have used it in root beer for mouthfeel. Otherwise, I'd leave the beer alone and do what you suggested and add the correct ingredients (grains) next time. I'm not a carapils user, but many people use it and like the results.
Here's the results of a quick experiment I did recently with Lactose, Dextrin and table sugar.
The rule was that table sugar, which was the sweetest of all, worth 100 points. We then compared the other two with table sugar. I mixed each ingredient with same amount of water. Can't remember but something around 4oz.
I got my wife and kid to help tasting the samples:cross:
Basically, dextrin tastes nothing, lactose taste a little sweet, but much less than table sugar.
Also interesting is that lactose sweetness is diferent than table sugar. I have no words to explain but lactose is a cold sweetness and table sugar is warm .