[wrholt]

I have an opinion on your first question, and why "mostrara" (imperfect subjunctive) rather than "muestre" (present subjunctive) or "mostró" (preterite): it will be interesting to learn what others think, especially native speakers.

The use of the word "que" introducing "mi madre mostrara..." is significant: a better translation of that entire clause is "(the fact) that my mother would show that so-visceral reaction upon mentioning my grandmother to her".

The writer wasn't asserting that their mother did it (simple statement of fact about a past event, objective), they were wondering (in the past) why their mother would do it (the topic of an opinion or hypothesis, rather than a statement of fact), which is consistent with using imperfect subjunctive.

There is a second possibility, depending on how long ago the book was written and how old-fashioned the writing seems to be in general. In earlier forms of Spanish "mostrara" wasn't imperfect subjunctive, it was pluperfect indicative with the same meaning as saying "había mostrado". Over the course of the past few centuries the usage of "mostrara" has changed so that currently it almost always functions as imperfect subjunctive. Journalists still use it with pluperfect meaning at times, but that's the only type of contemporary writing where you're likely to see that usage. So, if the book is relatively modern, it's much more likely that the writer has used "mostrara" as imperfect subjunctive.