That is a considerably involved recipe! I've been a home baker for over 40 years, and even I wouldn't attempt such a complicated recipe. Unless you are a very experienced home baker, you probably should give up on that one and find a simpler recipe. Most basic cookbooks have recipes for Boston Cream Pie.

You probably have a combination of problems that produced the failure. For one, the recipe relies on eggs for the leavening, instead of traditional baking powder. The slightest mistake in beating them might cause a collapsed layer. This might also cause the tunneling. Besides that, heating them first creates a whole other range of possible problems that could compound the beating issues.

It also sounds as if your oven temperature may not be accurate. An overly hot oven can cause a collapse, as it will dry out the cake and make it hard. An oven that's not hot enough can also cause the cake to fall, but it's less likely to become hard in that case. Have you checked lately to see if the oven is calibrated correctly with a reliable oven thermometer?

Hope you can figure it out. It's so much fun to please people with their favorite baked goodies, and so disappointing when it doesn't turn out well.