In my humble opinion, it seems that intercourse by way of the limbs is permitted occasionally according to Tosafot, so that would be preferable to having relations during the average expected period.

Intercourse by way of the limbs means ejaculation somewhere on the woman’s body. I saw a survey here: https://merkazyahel.org.il/%D7%91%D7%99%D7%90%D7%94-%D7%93%D7%A8%D7%9A-%D7%90%D7%91%D7%A8%D7%99%D7%9D-%D7%91%D7%91%D7%A0%D7%99-%D7%96%D7%95%D7%92/

If the emission of seed is caused by the wife, isn’t that considered by way of the limbs and therefore permitted?

In my opinion, no. That is masturbation, not intercourse.

Chelkat Mechokek (Even HaEzer 23:1, in the name of Sefer Hasidim) prefers wasting seed to violating the prohibition of a menstruant woman or a married woman forbidden to another man. Does that apply only to Torah-level prohibitions, or perhaps can it also be extended to a rabbinic prohibition such as the expected menstrual period?

And furthermore, there are opinions that it is not considered “in vain” when he is unable to impregnate his wife anyway (during times of prohibition and the like); can that be combined here?

On the other hand, there are halakhic decisors (including the Shakh) who hold that the average expected period refers only to the day of the month (excluding day 30 after a full month, and day 31 after a deficient month), or according to those who hold that only the actual expected period is forbidden and not the entire twenty-four-hour period.

Should one take any of these into account in a pressing situation?

I wrote that I do not deal with the question of what is preferable, because both are forbidden. The prohibition close to the expected menstrual period is rabbinic. That is all.