No question that "championship season" in the ML Rules refers to the regular season. It absolutely does, even though the cynic in me says that referring to the regular season as the "championship season," when 12 teams go into a post-season tournament to determine the "champion" smacks of 1984.
But that's why I read 7(a) as overriding 7(b) here - and that would mean that the 10/15 limit no longer applies - and that Gilbert and Stevenson could (and should) have been recalled as of two days ago.
Postseason player eligibility is governed by Rule 41(a)(1). To wit:
Nick Nelson doesn't meet requirement 41(a)(1)(A), since he was not on the 40-man, or any cited list, on August 31st. As written, the "substitution for injured player" loophole only applies to players who qualified under 41(a)(1)(A), but were "unreserved" after August 31st (that clause conditions 41(a)(1)(B), but does not affect 41(a)(1)(A)).