It depends on what one defines as pshat. I would say that many meforshei Rashi (Gur Aryeh as an example) assume that Rashi is always (or most often) saying peshat, even as he brings downs thousands of midrashim. And so, the sorts of difficulties / irregularities encountered in derash are precisely those which must be addressed in peshat.
However, other proponents of peshat (to whom I personally incline) will simply assert that these represent the natural variance of language, idioms, etc., such that one should not make a diyuk. See Ibn Ezra on parashat Yitro regarding the aseret hadibrot, at length, on this. part one, two, three, and four. From part three:
And know that the words {lexical items} are like bodies and the meanings are like souls, and the body is as a vessel to the soul. Therefore, the rule of all the scholars of every language is to guard to meanings and not pay particular heed to changes in the words, since they are identical in their meaning.
...
And many others you will find with different words, yet the meaning is identical. And as I said to you that sometimes they choose the short way and sometimes the long way, sometimes that will add a helping letter or to omit it, while the meaning is the same.
Hashem said: And techelet and argaman; and Moshe said: techelet and argaman. Hashem said: Avnei shoham; and Moshe said: and avnei shoham.
And there are many like these, and both of them are correct. For the Scripture without the vav chose the short way, and does no damage. Also the Scripture with the vav does no damage, since it added so as to explain. And behold, the vav, which appears in the pronunciation of the mouth, no man seeks for himself a reason why it is missing and why it is written, and why it is added. For both this and that are correct. And behold, it is apparent that the one who pronounces it would not seek upon it, when written, a reason. If so, why should we seek a reason for the omission of that which is not even pronounced. Such as, in the word le'olam, why it is written plene (with a vav) or deficient (without)? And if we were to seek a reason for only one of them (malei or chaser), or if the general custom was to write all of them in one way (malei or chaser), I would have remained silent.
And behold, I will give you an allegory. A person says to you, "Write to my friend, and this is what you should write: I, Ploni love you forever." And he writes Ploni, פלני, without the vav {which is acceptable in Hebrew}, and אהבך, 'I love you', also without the vav. And לעלם, "forever" also chaser. {In each case you can have a cholam chaser as opposed to a cholam malei.} And then Reuven comes and asks me, "why did you write them deficiently {chaser}? And meanwhile, I only have need to write that which he said to me, and I have no desire that they be either malei or chaser. Perhaps Levi will come and inform me how I shall write. I do not wish to go on at length. Rather, the intelligent person will understand.
As you can see, Ibn Ezra does not feel that one should make such diyukim on the level of peshat. Read inside to see if Ibn Ezra would then say that one should then turn to derash for the true interpretation.