The Tibetan ö is exactly like the High German one. In fact it's formed in a manner analogous to Germanic
umlaut: a back vowel is transformed into the corresponding front vowel, or a front vowel moves closer to i, due to the influence of a following sound. In the case of Tibetan, these sounds are final -d, -s (which are always silent in standard Tibetan), -l (which is pronounced in some words, but not in others), & -n (pronounced normally).
As for g & k, in standard Tibetan (my name for what exiles in India/Nepal speak as a lingua franca amongst each other, not the same as Lhasa Tibetan or "Central" Tibetan), unlike English & other European languages, the main distinction isn't between voiced vs. unvoiced, but aspirated vs. unaspirated, & high vs. low tone. At the beginning of a word,
ཀ་= unaspirated
ཁ་=aspirated high tone
ག་= aspirated low tone, slightly more voiced. Same with the
ཅ་ཏ་པ་ series. At the end of a syllable, as in
bdag, these distinctions don't matter, and as MrDistracted said it's "in between a k & g" (& in between a p & b).
'byung is future & present tense,
byung past tense, so another insignificant variant. (different tenses are extremely common in different editions of the same text, because they're often homophones when someone recites it for a scribe to carve the woodblocks)
Loppön Sonam Tsemo's account of Sachen's vision ends at
ས་མྰ་སྟ་མི་ཐི།, a misprint for Sanskrit samāptam iti "finished, end of quote". Jestün Dragpa Gyaltsen's instructions follow immediately after.
A
shad doesn't have to mark the end of a sentence, it's often just the end of a phrase. In this case the "sentence" ends at
མངོན་སུམ་དུ་གཟིགས་ཏེ།: "When Sachen was twelve ... he saw manifest before him." Of course the
te, a
lhag bcas (continuative) particle indicates that the thought is incomplete, and more's to come, so even this isn't a complete "sentence" in Tibetan. In fact the sentence is only grammatically complete at
ངེས་ཤེས་ཁྱད་པར་ཅན་ཐོབ་པ་ཡིན་ནོ on line 4 "... he obtained extraordinary certitude."
To translate it as a single English sentence, showing how the phrases are linked in Tibetan:
When Lama Sakya Chenpo was 12 years old, because he had done a six month
drubpa of Ārya Mañju, at one time he saw manifest before him in the center of a mass of light, on top of a jewelled throne, Lord Mañjughoṣa, orange in color, displaying the mudra of explaining the dharma, seated in bhadrāsana, surrounded by two bodhisattvas to his right and left; by investigating the meaning of what was said from the mouth of the central deity: "If you are attached to this life, you are not a practitioner of dharma, if you are attached to the three realms of existence, it is not renunciation, if you are attached to your own benefit, it is not bodhicitta, if grasping has arisen, it is not the view", he realized that all the practices of the 6 Transcendences were included in the mind training of the Zhenpa Zhidrel, and he attained an extraordinary certitude concerning all the teachings.
Clearly, just like English particle names aren't very useful for interpreting Tibetan, neither is English sentence structure. Learn to look for phrases and how they are linked, not sentences. Usually this is done with verbs + adverbial particles. This is a skill that comes with reading, not through studying grammar.