No, Maareecha taking the form of a deer in Ramayana not an interpolated story.
I am going to prove it in the following three steps.
1) Analyzing Knowledge of Rama, Lakshmana, Hanuman and Sita regarding Maareecha taking the form of deer from the event of Sita liking deer to the event of Hanumamn talks with Sita
2) Interchange of knowlede regarding Maareecha among Rama, Lakshmana, Hanuman, Sita, Ravana
3) Interpretation of Hanuman's statements
Ravana knows that the deer is Maareecha because Maareecha executed the plan of Ravana only.
Lakshmana believes that the deer is Maareecha
But Lakshmana became incredulous on seeing it and said to Rama, "I
believe this deer to be that Maareecha, the demon." [3-43-5]
Rama came to know that deer is Maareecha after killing Maareecha
"This is the trickery of Maareecha which Lakshmana vouchsafed earlier,
that has indeed happened in that way alone, and the one whom I have
killed now is none other than Maareecha. [3-44-23]
Although she might hear the name of Maareecha from Lakshmana, Sita do not know with clarity that the deer is Maareecha because she was abducted away before
Rama or Lakshmana confirms it with clarity.
From the information available, we can infer that Ravana, Rama knows the fact for sure. Lakshmana may be sure about the fact and Sita's knowledge is ambigious regarding the fact that the deer is Maareecha .
Ravana did not meet Hanuman and tell that the deer is Maareecha atleast before Hanuman meets Sita.
Both Rama and Lakshmana did not mention that the deer helped during abduction is Maareecha from the time of killing Maareecha to the time of Hanuman talking with Sita (Slokhas you mentioned from Sundara Kanda). We can also notice that neither Rama nor Lakshmana did not explicitly use even the name of Ravana till it comes from the Hanuman's mouth.
Thus no one among Ravana, Rama, Lakshmana tell to Hanuman that the name of Rakshasa who took the form of deer and helped in Sita's abduction is Maareecha.
We also need to note that Rama never disclosed this information to anyone atleast till Sundara Kanda that the deer is Maareecha. Observe that Rama did not even use the name Maareecha while talking to Lakshmana even
सर्वथा रक्षसा तेन जिह्मेन सुदुरात्मना |
वदता लक्ष्मण इति उच्छैः तव अपि जनितम् भयम् || ३-५८-१३
"That impostor and highly impudent demon (Maareecha) who shouted loudly
as, 'oh, Lakshmana,' might have caused fear even in you in everyway.
Now we will interpret the words of Hanuman you are referring. The words of Hanuman you are referring are neither supporting nor denying the presence of Maareecha but many other statements across Ramayana is supporting the presence of Maareecha.
Let us consider the statements one by one
The part of the tenth slokha of the thirty one Sarga of the Sundara Kanda of the Valmiki Ramaya you referred is
वञ्चयित्वा वने रामं मृगरूपेण मायया |
It is the statement told by Hanuman to Sita. The literal translation is
deceived Rama in the forest by illusionary animal form
The meaning is
(Ravana) deceived Rama in forest by illusionary animal (deer) form (of Mareecha)
Which is not same as
Ravana deceived Rama in the forest by becoming illusionary animal form
The part of the another slokha of the thirty four Sarga of the Sundara Kanda of the Valmiki Ramaya you referred is
अपकृश्ष्य आश्रम पदान् मृग रूपेण राघवम् |
It is a statement told by Hanuman to Sita. The literal translation is
Took away from ashram by showing animal form to Rama
The meaning is
Ravana took away (Rama) from ashram by showing animal (deer) form (of Mareecha) to Rama
Which is not same as either
Ravana took away (Rama) from ashram by becoming animal (deer) form (of Mareecha) to Rama
Ravana took away (Sita) from ashram by becoming animal (deer) form (of Mareecha) to Rama.
Now, your argument is that Hanuman did not use the name of Maareecha explicitly and hence there is a chance for interpolation. But we can notice that from steps 1 and 2 that Hanuman does not have explicit knowledge on Maareecha and hence he might not use the name Maareecha.
Now, from the comments, the question is that how do Hanuman then know about the animal form taken inorder to decieve Ram to abduct Sita.
Let us observe the following slokhas by lamenting Sita, which was observed by Hanuman secretly
तरस्विनौ धारयता मृगस्य |
सत्त्वेन रूपं मनुजेन्द्रपुत्रौ | नूनं
विशस्तौ मम कारणात्तौ |
सिंहर्षभौ द्वाविव वैद्युतेन || ५-२८-९
"In the disguise of a animal, those two energetic sons of Dasaratha were
surely killed for my sake, as two excellent lions killed by a flashing
नूनं स कालो मृगरूपधारी |
मामल्पभाग्यां लुलुभे तदानीम् |
यत्रार्यपुत्रं विससर्ज मूढा |
रामानुजं लक्ष्मणपूर्वजं च || ५-२८-१०
"Certainly that Time-spirit, assuming the guise of a animal, allured
me, a woman of poor fortune at that time and to whom I, a stupid
woman, sent forth (far away) Lakshmana and the noble prince Rama the
elder brother of Lakshmana."
From the above slokhas, Hanuman can come to know that Sita sent the Rama and Lakshmana for the sake of illusionary animal form.
Along with these three steps, we can consider the following slokha:
Narada told to the Valmiki the following lines from the sarga 3 of Bala Kanda while saying Ramayana in sequence in concise form
The killing demons like Khara, Trishirasa and the upsurge of Ravana
thereby, and also killing demon Mareecha, and Ravana's abduction
of Vaidehi, likewise
Narada mentioned the Killing of Mareecha which happened only before the abduction of Sita. According to your argument if Maareecha does not involve in helping Ravana then the Killing of Maareecha should not be mentioned by
the Narada to Valmiki.