💚The Buddha went straight to Buddhahood without becoming a non-returner?

SN 12.10 appears to clearly describe the stream-entry of the Buddha-To-Be.

Iti 96 defines a non-returner simply as one that does not return to sensuality.

AN 3.86 says only non-returner & Arahants have completed concentration (jhana). MN 14 says when the Buddha-To-Be had mastered jhana his mind no longer could return to sensuality. Therefore, MN 14 appears to show when the Buddha-To-Be was a non-returner.

The following standard definition of a non-returner appears generally mistranslated:

In this teaching there are mendicants who have given up the five lower fetters. All of them are reborn spontaneously. They are extinguished there, and are not liable to return from that world.

Evaṁ svākkhāte, bhikkhave, mayā dhamme uttāne vivaṭe pakāsite chinnapilotike yesaṁ bhikkhūnaṁ pañcorambhāgiyāni saṁyojanāni pahīnāni, sabbe te opapātikā, tattha parinibbāyino, anāvattidhammā tasmā lokā.

The term 'opapātikā' does not mean 'reborn'. AN 3.141 says:

And how is an excellent person fast, but not beautiful or well proportioned? It’s when a mendicant, with the ending of the five lower fetters, is opapātiko. They’re extinguished there, and are not liable to return from that world. This is how they’re fast, I say. But when asked a question about the teaching or training, they falter without answering. This is how they’re not beautiful, I say. And they don’t receive robes, almsfood, lodgings, and medicines and supplies for the sick. This is how they’re not well proportioned, I say. This is how an excellent person is fast, but not beautiful or well proportioned.

The term 'parinibbāyino' appears to not refer to Nibbana. Instead, the term 'parinibbāyino' appears to merely refer to the complete extinguishment of the five lower fetters, by non-returning.

The term 'lokā' merely refers to mental states, such as in MN 79, where jhana is called "a world that is exclusively happy". AN 4.45 says:

For it is in this fathom-long carcass with its perception and mind that I describe the world, its origin, its cessation, and the practice that leads to its cessation.

Api cāhaṁ, āvuso, imasmiṁyeva byāmamatte kaḷevare sasaññimhi samanake lokañca paññāpemi lokasamudayañca lokanirodhañca lokanirodhagāminiñca paṭipadanti.

In summary, SN 12.10 shows when Gotama attained stream-entry. MN 14 reports a time when Gotama was a non-returner. Regardless, the term 'non-returner' merely refers to not returning to sensuality & the five lower fetters. It does not mean not being reincarnated again on Planet Earth.

Based on the Dharma sermons I've heard & read. If you mean "Anagami" — the 3rd stage of awakening — by "a non returner", then yes, all non-returners finish their awakening in the Brahma Realm. They are never reborn as humans, deva or other types of beings.

There are 31 Realms and all humans who achieved non-returner stage are born in Suddawasa Brahma Realm or ones above it.

Followers and others who are within the 4 stages of awakening can't become a Samma Sambuddha. BUT they can become a Buddha as the person who broke the cycle of being born again after reaching Arhat — the 4th stage of awakening. It's one of the meanings for the word Buddha.

I'm not sure the exact answer to your 3rd question, but I have seen various people who attained Arihat, Anagami, Sakrudagami and Sowan by just 1 Dhamma Sermon via the Tripitaka Sutras. So I think depending on the level of wisdom (Pragna) of the person, they can jump straight into a specific stage, but they still go through the stages from 1 to 4.

See the 3rd paragraph above on how we can consider Arahant as Buddha, but not as Samma Sambuddha.

I'm not sure the answer to your 5th question also.

Lord Buddha traveled to Deva Realms from the Human Realm when he taught the Abhidharma for the beings above the Human Realms.

Hope this answers your questions!

The Buddha explained in the sutta quote below that he attained the knowledge of the four noble truths and became completely liberated, all in one night.

On the other hand, a stream enterer is one who learned from a Buddha and has completely understood the four noble truths through insight, but still has seven more fetters to overcome before becoming fully liberated.

As such, we can conclude that the Buddha did not go through the four stages to become liberated. Rather, he attained it all in one go.

A Buddha attains the knowledge of the four noble truths only after fully overcoming ignorance. A stream enterer learned the knowledge of the four noble truths from a Buddha, then overcomes ignorance later.

"When the mind was thus concentrated, purified, bright, unblemished, rid of defilement, pliant, malleable, steady, & attained to imperturbability, I directed it to the knowledge of the ending of the mental fermentations. I discerned, as it had come to be, that 'This is stress... This is the origination of stress... This is the cessation of stress... This is the way leading to the cessation of stress... These are fermentations... This is the origination of fermentations... This is the cessation of fermentations... This is the way leading to the cessation of fermentations.' My heart, thus knowing, thus seeing, was released from the fermentation of sensuality, released from the fermentation of becoming, released from the fermentation of ignorance. With release, there was the knowledge, 'Released.' I discerned that 'Birth is ended, the holy life fulfilled, the task done. There is nothing further for this world.'

"This was the third knowledge I attained in the third watch of the night. Ignorance was destroyed; knowledge arose; darkness was destroyed; light arose — as happens in one who is heedful, ardent, & resolute.
MN 19

Here's an analogy.

The Buddha is like Einstein who discovered the Theory of Relativity. He's the only one, and there's no one else like him.

The stream enterer is the physics bachelor's degree graduate who studied relativity and passed. He has proven understanding of the breadth of physics, but not the depth. He has entered the physics academic stream.

The once returner is the physics master's degree graduate. He needs to return to university one more time to do his doctorate.

The never returner is the physics doctorate graduate. He doesn't have to return to university again to do a degree, but he still needs to build up his research portfolio and experience.

The Arahant is the tenured physics professor. He has reached the peak of his career.

Einstein obviously never studied relativity for his degrees because he is the one who discovered it and instantly became the first professor of it.