So I think you got the general gist of the sentence. But, you translate:
one beer as in binge-drinking
I'm not sure why you said that, but the の just acts in its normal fashion. I think you should consider it as something like
one beer of binge-drinking
Then the parenthetical comment of とも言えないが can be understood as something like
Indeed both are possible due to the potential form of いただく heading the verb.
That said, I think が is very slightly more natural here, because you don’t really want to focus on the customer and the fact they can choose drinks (which feels very slightly less polite), but rather the drinks and the fact they can be chosen by the customer.
I just used the ...
Your suspicion is entirely correct. The が marks the potential verb.
I think I would personally view お選びいただけます as a single (albeit compounded) verb, just as a I would view 選んでもらえる (which holds the same meaning) as a single (but again compounded) verb, so maybe that's a helpful way to think about this sort of construction taking が?
That all said, the use of ...
Also depends a lot of the reason why (and hence the tone in which) the speaker says すみません or すみませんが. I think it is a bit risky to use すみませんが if you simply want to be polite, as it could be taken as the hostile すみませんが (used to attrack attention when you want to point out something that you are not satisfied with) even if you (like me, a foreigner) tried to ...
Literally, the が here means “but” (in this case it is not used as a particle). Polite or humble Japanese often uses が or けど to end subclauses. Similarly, passive or negative verbs are used as it is considered more polite to be indirect.
Excuse me but could you please ~
In this case すみません (which has many different uses) is used to get ...
At the very core of your sentence, the parsing is as follows:
血糖値が[Number]しか上がらなかった。The blood sugar only rose by [Number].
All the other stuff is just details which modify that core statement in some respect. Let's add in some extra details to the core statement.
血糖値が７７ミリグラムしか上がらなかった。 The blood sugar only rose by 77mg.
Now you can add in a clause ...
の can be used in place of が in relative clauses, although there are some exceptional rules. The basic rule of ga-no conversion is described in this question: How does the の work in 「日本人の知らない日本語」?
When ga-no conversion happens, が and の are almost the same in modern prose, but some sources say the use of の is gradually declining. I personally feel が sounds ...