Why can’t TTG’s Allowable Changes result in manipulations carried to extremes?

Because all of those changes are judged by rinairs in P7.

If any those changes are carried to an untrustworthy extreme, the result won’t meet P7 and can’t qualify as TTG.

What about manipulations not allowed by rinairs and TTG?

For those manipulations there is no limit, since rinairs doesn't apply and there is no other way to set limits that are not arbitrary.