All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Before answering all the points you asked about in your question, it should first be noted that the religious ruling which we think is the preponderant opinion about the matter, is that if the pregnant woman and the breastfeeding woman break the fast in Ramadan without travelling or being in menses or post-partum bleeding, then this could only be due to the following three reasons:
The scholars differed in opinion about these cases but the most preponderant opinion –Allaah knows best –is the opinion of Hanbali and Shaafi'i Schools of jurisprudence, which states that if they feared about their babies only and they broke the fast, they should make up the fast and feed a poor person [for each day], and if they feared about themselves only or about themselves and about their babies, then they should only make up the days they broke the fast.
Therefore, the pregnant woman and the breastfeeding woman are considered like a sick person and a traveler, so they are only obliged to make up for the days they broke the fast. The fact that Ibn 'Abbaas and Ibn 'Umar did not mention making up the missed days (in this case) is because this is a well-known matter and there was no need to mention it. Concerning the Prophetic narration which reads: "Allaah Has exempted the pregnant woman and the breastfeeding woman from fasting," then what is meant here is the obligation of observing the fast, but they are obliged to make it up as there is no clear expression which suggests that they are not obliged to make it up.
As regards your second question about the view of Ibn Taymiyyah about the matter, then we have not come across his view about this matter.
With regard to the third question, there is no doubt that the companions are the best people of this nation after the Prophet and there is no doubt that the two companions whom you mentioned in the question are among the most knowledgeable companions, and the Prophet supplicated for Ibn 'Abbaas with the supplication which you mentioned in the question. Despite all this, it is not strange that Shaykh Ibn Baaz and Ibn 'Uthaymeen considered their view as weak, because all people’s sayings could be accepted or rejected except those of the Prophet, sallallaahu ‘alayhi wa sallam.
The scholars differed in opinion about the issue of considering the statement of a companion as considerable evidence; some consider that their statement is considerable evidence but this does not mean that the companion is infallible. The statement of a companion is evidence as long as there is no stronger evidence which is more preponderant than his statement.
Moreover, if we say that it is not permissible to go against the statement of a companion, then what to do about their difference of opinion in one single issue, and it is known that they differed in many branches?
Concerning your last question, in cases of a difference of opinion, it is an obligation on a Muslim to act according to the most preponderant opinion of the scholars . Therefore it is not permissible for your wife to act upon the view which says that it is enough to feed a poor person for each day as long as she is able to fast while she is convinced that it is not the preponderant opinion.
Allaah Knows best.