To the first half of this ask, essentially, yes. Blissey eggs are simply giant eggs, and whatever you can do with ordinary chicken eggs, you can do with blissey eggs. However, most trainers prefer having their blissey use Softboiled and then serving the newly literally soft-boiled egg over freshly made, hot rice. This method “activates” the egg (in that the blissey imbues it with further healing properties) and slightly cooks it at the same time. Thus, it’s less work for the trainer and twice as potent as normal.
Although the above is the preferred method, non-activated eggs are also effective at curing sadness. In fact, a lot of trainers prefer cooking their egg to allowing their blissey to use Softboiled, as this provides more flexibility. (You can’t un-Softboiled an egg, so if you prefer your eggs any other way but soft-boiled, you’re out of luck if you use the above method.) However, it’s important to note that the more ingredients you add to a blissey egg, the more diluted its effects will be, so while pancakes made with blissey eggs are, from what I understand, both delicious and capable of making one happy, they won’t be as effective at lifting one’s spirits as blissey eggs sunny side up.