The general idea of this sentence is that the Summa logicae was a kind of source text for other, later logic texts. (They "owed a great debt" to the work.) Whenever something is called a "seminal" work, it is an important source in this way. Actually, the word "seminal" comes form the Latin word for "seed." Hence, seminal things are like "seeds" that grow into other things that bud forth from them. The Latin word semina has many derivatives in English. "Insemination" is the process of making someone pregnant; it is the process of placing the male "seed" into the female egg. (Granted, the idea of "male seed" is certainly a bit of old-fashioned Aristotelian biology.) Likewise, as "seminary" where Christian clergy are trained actually means seedbed—as though the people being trained are like seeds that will grow into fully mature clergy members. 

As for the second blank, you are just looking for a synonym for "weighty." The word "substantial" is a fine synonym in this way. Think of it as just meaning "it has a lot of substance." This is really all that we can say: the text was no small affair!

The first blank is actually quite easy, so long as you pick up on the correct clues. The story about the tower is not really true. It is a kind of "tall tale." While it does seem that this was an interesting story for the visitors, it is better to express the idea of the tale being based on doubtful information. This is what it means for something to be apocryphal. Since even the monk did not know that his story was false, we can guess that it had arisen from some apocryphal sources. At least among the options provided, this is the best word.

The people were apparently naive in believing the tale. Hence, the word "gullibly" expresses the fact that they so willingly believed the story about the so-called "sauerkraut tower."

"Tawdry", meaning cheap looking, works because it describes the cheap look for the notebook that came from a dollar store. "Frugal" works because it describes the mom who seeks out cheap things.

"Desiccated" works because it describes an environment completely without moisture; "uncomfortable" works because it describes the infant who needed appeasing.

"Prurient", meaning interested in sexual matters, works here because the teens are engaged in lewd talk. "Nascent", meaning new, works here because the physical education class just started this year.

"Mellifluous", meaning sweet and melodious, works because it describes the singing birds. "Hungry" works as it provides reason for the cat eating the birds.

"Fiery" works because it describes the short temper of the babysitter. "Rambunctious" works because it describes children who never calm down.

"Rabid" works as an adjective because it provides a reason for why the dog would be foaming at the mouth. "Frightened" works as a descriptor for the veterinarian, as the following phrase alludes to his fear.

"Troubled" works as an adjective for the youth, who has been to jail many times and seems to have problems. "Awakening" describes the change the youth experienced after his friend went to prison, and the subsequent turn around he experienced in his life. 

"Enchanting" works because it matches the description of the witches' elegance from the following sentence. "Unsuspecting" works because it describes the sailors, who didn't know the witch was dangerous.