A בכור and a פשוט, which the father left over for them, etc. - בכור ופשוט שהניח להן כולי
According to the ר"ת the question was that if we divide it by days the בכור will receive more than double, the answer is that he is entitled to it. According to the ר"י the question was he will receive less than double, and the answer is that he will receive double regardless.
As it is written; he did not create it to be desolate, etc. - שנאמר לא תהו בראה וגומר
The advantage of לשבת over פו"ר is: a) that it applies even to the צד עבדות, as opposed to פו"ר, or b) that (only) שבת is a מצוה רבה. We (generally) cannot force the אדון to be משחרר an עבד so that he should become מחויב in a מצוה . A woman is not obligated for פו"ר (but is obligated for שבת).
תוספות first explains why the rule of עדל"ת cannot allow the חצי עבד marry a Jewess. תוספות continues to explain why the rule of אין אומרים לאדם חטא כדי שיזכה חבירך does not apply to the master. תוספות explains why this חצי עבד cannot be מקיים פו"ר in other ways. תוספות concludes with the laws concerning the intermarriage between various פסולים.