In Ramayana, Vibhishana was wise enough to know that Ravana was doing adharma and therefore refused to support him in the war. But did Kumbhkarana and Meghnada know that they are on the side of adharma? If they were aware, what was their obligation to fight from Ravana's side? (For example, in Mahabharata, Karna had a proper justification for supporting Duryodhana)
Kumbhakarna knew that Ravana was in the wrong. On being called to aid during the war, Kumbhakarna mocks Ravana’s sins by saying:
दृष्टो दोषो हि योऽस्माभिः पुरा मन्त्रविनिर्णये। हितेष्वनभियुक्तेन सोऽयमासादितस्त्वया॥२॥ शीघ्रं खल्वभ्युपेतं त्वां फलं पापस्य कर्मणः। निरयेष्वेव पतनं यथा दुष्कृतकर्मणः॥३॥
A swift retribution has followed on the error of judgment we saw you commit formerly in council through lack of trust in your ministers. Just the way a sinner is destined to fall into the hells, the fruits of your sin has approached you very fast.
Srimad Ramayana, Yuddha Kanda, Chapter 63, verses 2-3
The sin referred to above is the sin of kidnapping the Goddess.
However when Ravana rebukes Kumbhakarna by saying that bygones are bygones and being a brother, Kumbhakarna must help Ravana in distress. Kumbhakarna expresses his willingness to go to war by justifying as follows:
सदृशं यच्च कालेऽस्मिन् कर्तुं स्नेहेन बन्धुना। शत्रूणां कदनं पश्य क्रियमाणं मया रणे॥३४॥
Whatever is befitting for a brother at this hour, out of love, see that being done by me i.e. destruction of the enemies.
-Srimad Ramayana, Yuddha Kanda, Chapter 63, verse 34
So though he knew Ravana had sinned, it is out of brotherly affection (to help his brother in distress) that Kumbhakarna goes to war.
On the other hand, Indrajit is too boastful of his prowess and seems to be supporting whatever Ravana did. He does not seem to have the wisdom that Kumbhakarna, Vibhishana, and Mandodari had. This is evidenced by the fact that after Lanka Dahana, when Ravana has called his council for consultation, Indrajit boasts of his valour:
सोऽहं सुराणामपि दर्पहन्ता दैत्योत्तमानामपि शोककर्ता। कथं नरेन्द्रात्मजयोर्न शक्तो मनुष्ययोः प्राकृतयोः सुवीर्यः॥७॥
I who humbled the pride of the Gods themselves and afflicted the Daityas, am I in mine immeasurable energy not able to subdue these two princes, insignificant mortals as they are?
-Srimad Ramayana, Yuddha Kanda, Chapter 15, verse 7
And to this Vibhishana rebukes him by saying that Indrajit supporting Ravana’s wrongs actually shows that he is an enemy in the guise of a friend:
पुत्रप्रवादेन तु रावणस्य त्वमिन्द्रजिन्मित्रमुखोऽसि शत्रुः। यस्येदृशं राघवतो विनाशं निशम्य मोहादनुमन्यसे त्वम्॥१०॥
Under the guise of a son, O Indrajit, you are in truth a hidden foe to Ravana and, hearing him prate of slaying Raghava, you dost support him.
-Srimad Ramayana, Yuddha Kanda, Chapter 15, verse 10
Since Indrajit supported Ravana, there is no question of any justification required on his part to fight from Ravana’s side.
Since the question is only on Kumbhakarna and Indrajit, I am not dwelling upon the Vibhishana aspect.
Kumbhakarna advised Ravana to follow Vibhishna's advice:
यदुक्तमिह ते पूर्वम् क्रियतामनुजेन च | तदेव नो हितम् वाक्यम् यदिच्छसि च तत्कुरु || ६-६३-२१
"That advice which was tendered to you earlier by Vibhishana is beneficial to us. Do whatever you wish to do."
At least in Valmiki Ramayana Indrajit never advised surrender to Ravana, although Doordarshan seems to think so: