On Rig Veda 4.26.1 (quoted in Brhadaranyaka 1.4.10):

अहं मनुरभवं सूर्यश्चाहं कक्षीवाँ ऋषिरस्मि विप्रः ...

This is Sāyaṇācārya's commentary on the RV mantra:

अत्र ’अहं मनुः’ इत्याद्याभिस्तिसृभिरात्मानमिन्द्ररूपेण वामदेवः स्तुतवान् । गर्भे वसन् वामदेवः उत्पन्नतत्त्वज्ञानः सन् सार्वात्म्यं स्वानुभवं मन्वादिरूपेण प्रदर्शयन्नाह । ... परमार्थदृष्ट्या कृत्स्नमप्यहमेवास्मीत्यर्थः ।

Here, by the triad beginning "I was Manu", Vāmadeva praises himself in the form of Indra. Being in the womb, having realized the supreme knowledge experiencing everything as his Self, he says that he was Manu etc... The implication is that he is saying that he is everything (i.e. as Brahman).

In other words, as the Upanishad quoting this mantra shows the context, Vāmadeva has realized that he is Brahman. Because of that, he is metaphysically speaking as Indra (who is also Brahman).

On Rig Veda 4.27.1 (quoted in Aitareya 2.5):

गर्भे नु सन्नन्वेषामवेदमहं देवानां जनिमानि विश्वा ...

This is Sāyaṇācārya's commentary on the mantra:

गर्भे एव विद्यमानः अहं वामदेवः एषां देवानां सर्वाणि जन्मानि अवेदम् । परमात्मनः सकाशात् सर्वे देवा जाता इत्यवेदिषमित्यर्थः । इतः पूर्वं बहूनि अभेद्यानि शरीराणि मामपालयन् । यथाहं शरीरात् व्यतिरिक्तमात्मानं न जानीयां तथा मामरक्षन्नित्यर्थः । अधुना श्येनवत्स्थितः वेगेन शरीरान्निरगमम् । अनावरणमात्मानं जानन्निर्गतोऽस्मीत्यर्थः ।

When I, Vāmadeva, was in the womb, I knew of all the births of the deities, i.e. I knew that they are all forms of the Paramātman. Earlier, I was held down by many impenetrabe bodies, such that I could not recognize the Ātman as separate from the body. Now, like a swift hawk I have escaped the body, i.e. I have realized the unrestricted Ātman.

So clearly, this mantra is also expressing in poetic symbolism the self-realization of Vāmadeva.

There are many such examples in Vedas where a self-realized rishi speaks in the first-person as a deity. For example, Rig Veda 3.26.7 is Vishvamitra's self-realization, which he expresses as "I am Agni, omniscient by birth, etc".

It is amusing to see such an ostentatious bombastic display of verbosity and "scholarship" by busy quoting and cross-referencing and linking and such, bringing in irrelevant points, all for the purpose of over-complicating it (or making it look pseudo-sophisticated?).

These two metaphors are very simple and straightforward to understand, if there are no prior assumptions clouding the understanding.

The question contains prior assumptions - incorrect statements that are written as if they were fact. There is a lot of projection of post-Vedic ideas on the Vedic mantras:

1. "Two of the hymns of this Mandala deal mainly with Garuda's theft of the Soma/Amrita (nectar of immortality) from Devaloka" -- This is incorrect projection of Puranic Garuda on Vedic mantra. No commentator has mentioned "Garuda" or "theft of Amrita from Devaloka". The symbolism of the hawk is very different in the mantras.

2. "I was Manu (moon)" -- Manu does not mean "moon". LOL. The link to the English translation has this, and it is completely incorrect. So much for over-reliance on English translations rather than authentic, native, traditional commentators.

3. "Again, it seems like the speaker in these verses is Chandra," -- Another assumption without proper sources. But also, what purpose does this speculation serve?

4. "The authors of the newly-released Oxford English translation of the Rig Veda also believe that the Anukramani misinterpreted these verses" -- It is disturbing, to say the least, that western academics, who are not steeped in Vedic tradition and who have been proved repeatedly to misinterpret mantras based on their own theories, are trusted to pass judgement on a traditional Vedic text that has been passed down for thousands of years through teacher-student lineage whose life-long mission was to immerse themselves in sincere study and understanding of the ancient knowledge from the time of the rishis.

I don't mean to offend anyone, but I have seen this user consistently post questions with overly elaborate writing display when a simple short question would convey the message (such as "what do these mantras mean according to tradition?").