I don't know how your strings are formatted but it should work this way:
<string name="your_string_id">First line\nSecond line</string>
</br> is for HTML content.
Another option could be:
yourString= yourString.replaceAll("\n", System.getProperty("line.separator"));
The other question its a little more difficult, you have to create your own InstructionStep Implementation:
public class YourInstructionStep extends Step {
@Override
public Class getStepLayoutClass() {
// Here is your custom layout it should extend FrameLayout and implement StepLayout
// This class must override the getContentResourceId() method to define the layout.
// You can add whatever you want inside that layout.
return YourInstructionStepLayout.class;