1. The text is on the UI layer with parallax = 0, so you could just centre it inside the dotted line if you want it to appear in the middle of the screen.
<img src="https://dl.dropboxusercontent.com/u/8367729/construct/pics/gumbo1.PNG" border="0" />
If you to position it at the player coordinates you need to convert them with LayerToCanvasX(layer, x, y) and LayerToCanvasY(layer, x, y).
<img src="https://dl.dropboxusercontent.com/u/8367729/construct/pics/gumbo2.PNG" border="0" />
2. Mmm no idea, looks like you might have found a bug. When it happens the debugger shows the object count going into negative numbers.
<img src="https://dl.dropboxusercontent.com/u/8367729/construct/pics/gumbo3.PNG" border="0" />
Seems to be connected to the Sparkle particles spawning.
3. You're spawning the explosion on layer 0, the background layer with parallax 0,0.
4. Same.
5. You shouldn't have the background parallax set to 0,0 because it won't scroll and you won't see anything outside of the dotted line.