Is it necessary for a woman with istiḥāḍah to perform ablution for every prayer after its time has begun? Is the aforementioned apparent combination permissible, given the permissibility of a real combination? And is it permissible for her to combine prayers with a single ablution?

What you have mentioned regarding the rulings is correct. Perform ablution for every prayer if anything exits. If she performs ablution before the time, her ablution is valid as long as nothing exits; otherwise, she must repeat the ablution. The woman with sustained non-menstrual bleeding (mustahadah) performs ablution for the time of each prayer, and she may combine two prayers with a single ablution.

The mustahadah performs ablution at the time of prayer, prays the obligatory prayer, makes up missed prayers, and performs voluntary prayers until the time expires.

It is permissible for the mustahadah to combine two prayers in a "pictorial" manner (jam' suri) with a single ablution, even if the time has exited. However, it is more cautious for her to perform ablution for each prayer.