Should one - moreover can one - defrost a frozen lasagna before cooking?

Frozen foods are made for convenience. For most people, convenience does not include remembering to defrost the food several hours before cooking. So the recipe and cooking method are indeed for cooking straight from frozen, unless they clearly state otherwise. You will get the best result if you take it directly from the freezer to the oven.

For most frozen foods, defrosting will make little or no difference. You would need to cook it for less time than in the instructions to avoid overcooking, but the end result should be about the same. Occasionally, defrosting before putting it in the oven will decrease the quality considerably. This is in particular with frozen pizza and other foods involving a crust where the base will absorb a lot of liquid during a long defrost (as opposed to a quick one in the oven on a rack) leading to a soggy crust. However, as long as you follow a safe method for defrosting (in the fridge), it is not a food safety issue. The quality may be slightly worse, but it's perfectly safe to eat.

Outside of premade freezer dishes (as Johanna notes, those are basically never defrosted), the "do I need to defrost it" question is really quite interesting. For the most part, if the food is already in its "to be prepared" state, defrosting isn't very important. It's when you're going to further work with it that you need to defrost it.

The rule on defrosting for the most part is that it might be necessary if it's a problem for the food to cook very unevenly. A steak is a good example of this: assuming it is reasonably thick, like a ribeye, if you cook it from frozen, the inside will still be frozen when the outside is fully cooked, so you do want to at least slightly defrost it. Thin steaks are fine to directly cook from the freezer, though, and even the ribeye might be fine given some practice with temperatures - just harder to get that perfect inside temperature. Bacon, on the other hand, is thin enough that usually you don't need to defrost it if it was stored in individual strips (not that we ever remember to do that). Hamburgers do not need defrosting, again if packed individually and in already formed patties; their texture is such that "overcooking" the outside is often considered a good thing (ignoring the changes in meat texture from freezing in the first place, of course). My regular breakfast right now is chicken or salmon burgers with egg on an english muffin, and I cook those directly out of the freezer - in fact, the directions say to!

Things cooked in the oven are usually, if not always, not required to defrost (as they are slower to cook, and temperature will even out) - but they might require slight alterations to their recipe, and some care for the changes freezing brings. A lasagna, for example, probably cooks fine from frozen - but be careful that you don't have too much water in it once it thaws. An unbaked pie or similar is probably fine also, but you are more likely to need foil covering the edges to prevent burning as you'll be cooking it for much longer than a non-frozen.

In a microwave, defrosting before cooking is not generally needed. However, for foods of larger bulk, cooking on a lower power (which is what the defrost cycle does, for microwaves capable of actually modulating their power) or with on/off cycles (how other microwaves simulate lower power) will allow the food to warm more evenly; this might be helpful for food quality in some cases. If it's something you can stir, though, you're probably fine just going full blast and stirring in the middle of the heating cycle once or twice.

For other cooking methods... Cooking in a pressure cooker, you basically never defrost - it just takes a bit of extra time to cook. Same for Sous-Vide - as long as you're not cooking at too low of a temperature such that you end up in the danger zone, anyway. Cooking in a slow cooker ("Crock Pot"), though, should never be done from frozen unless the food is dispersed (i.e., a bag of frozen veggies is fine); because of the extremely low temperatures, you'll be cooking up a pot of bacteria along with your food.

Unless it has recently changed, or there are regional differences in the packaging, as I recall there are two sets of directions on the package... one for frozen and one for thawed.

Another "technique" is included which, for the sake of speed, calls for thawing in a microwave and then completing the cooking in an oven, too.

The biggest (only) difference is the cooking time. I've cooked them all three ways and... there's little difference in the finished result.