Welcome, Rob. nota bene: aliquid is neuter and cannot refer to a person, so it cannot mean "someone." I found this previous discussion
which might be helpful to you, and I'll do my best to answer the new questions you raise.
I typically break down the sentence into probable phrases:
B. vita illius modi aequi
C. aliquid iucindi atque felicis
This is an accurate breakdown.
(a) although (keeps, encloses, restrains, contains) all carry the same sense of limiting something, "contains" carries other senses, for example including something, as in "life contains ups and downs", and "keeps" also carries other senses, for example maintaining something, as in "life keeps death away".
The basic meaning of teneo is "to physically hold." In a figurative sense it can mean "to possess" or "to hold within oneself, contain"- the former meaning is where the words for "to have" come from in modern Romance languages. The word can also mean "to hold in place," but it doesn't often mean "to hold back, restrain," that sense being more common in compounds such as retineo or detineo. I think reinterpreting aliquid as a thing and not a person will make it clear that "restrains something" doesn't sound quite right and that the verb is likely to have another sense here.
(b) why would we choose "calm" over "just" or "favorable"? These seem to have very different senses.
The basic meaning of aequus is "level" or "even." I feel it's unclear from the context whether it should be translated as "just" or "calm" here. Besides that, the construction "illius modi aequi" is really clumsy, and I don't think a Roman ever would have said it, so please don't stress over that portion of the sentence.
flebile nescio quid queritur lyra, flebile lingua murmurat exanimis, respondent flebile ripae