Can past sense verb follow “in this moment”?

It is grammatically correct but does not show the best idiomaticity, in my opinion. Something such as the following would have seemed more usual to me.

• Now they've reached this moment [during which/when/where] they were planning to release a bill last week.

There is no difference as to meaning; "where" can be used to stand for places, situations and stages (Cambridge Dictionary). The particular moment mentioned can be identified to a given situation or stage in a progression. The given form used communicates somewhat different connotations. For instance the same general idea could have been written like so:

• "Now they are at the point where they were planning to release a bill last week.".