The liquor is useless for him as his body most likely developed a resistance to it. It is made apparent that being sick might be a prerequisite for the liquor to have an effect (when Conan instructs Haibara to drink it back in the Reunion arc, for example), but since neither of them have tried to drink it again, even when they were sick, it gives off the impression that the liquor itself is now useless for them. Haibara does use the liquor as a base for her antidote, though.
I looked around to see what people thought of anime openings largely spoiling things and it’s fairly mixed. One answer I saw a few times, although its veracity is seemingly up for grabs, is that since anime is an adaptation of a manga, studios go with the assumption that most of their viewing audience will be those who have already read that particular manga. Another notable response that I kinda agree with is, “is it really a spoiler if you don’t know the context behind it?” This one came up a lot in different flavors.
Others see the practice not as spoiling but rather to hype the viewer, tease them, what to expect, etc. Of course, all of these threads came from people asking about anime in general. I only found one thread in the Detective Conan subreddit that asked about this, and even that disregarded the preview sequence that you talked about. I am also not sure how other anime do this, since DC is the only anime I’ve watched (and will probably be the only one I’ll ever watch), so I do wonder what those people would think of DC’s openings.
With that said, it’s probably in your best interest to start recognizing when the opening song starts. Warning you now, there is an opening during the Bourbon Arc that immediately starts with the preview sequence. You won’t encounter it until the mid-600s, but just letting you know now.