Summary:
When the גמרא answered דקיימא באגם it was not limiting the liability of a שואל; it was only pointing out the strictness of קרן בלא שבועה over כפילא בשבועה.
However, according to the one who maintains, the Torah speaks in the language of men, etc. - אלא למאן דאמר דברה תורה כלשון בני אדם כולי
Summary:
A דרשה דלא מיסתבר, not only means that the rule derived is not logical, but it can also mean that the method of derivation is illogical (even if the rule is rational).
Summary:
יחדו by שעטנז וכלאים is necessary to exclude one תכיפה or not tied to the מחרישה respectively; ר' יונתן derives his rule based on logic, not from the ‘extra’ יחדו.
