IntraTextAdornmentManagerBase-based adornment managers like IntraTextNoteAdornmentManager are set up to watch for text ranges to be tagged with a specific tag type. In this case, IntraTextNoteTag is the tag type being watched. As soon as you tag a range of text with IntraTextNoteTag, the adornment manager will find the tag and call AddAdornment.
In the AdornmentsIntraTextNotes sample's MainControl.OnAddNoteButtonClick, you can see how a tag is created and added to the tagger. That's all that needs to be done, since it triggers AddAdornment to be called.
As long as the IntraTextNoteSyntaxLanguage is set up with the two services listed in its constructor, everything should work. You shouldn't need to create an adornment manager in the MainControl.
For sizing intra-text adornments, that sample's IntraTextNoteTag class has Size and Baseline properties. Those would need to be set appropriately and can affect the height of the overall viewline. SyntaxEditor will try to alignt the baseline of our own rendered text with the baseline you give for the adornment.
For the multiple lines of code with indentation question, perhaps an intra-text adornment is not the best option in that scenario. For that, you might want to do an intra-line adornment. There are a couple samples on that kind of thing and documentation.