Shouldn't it be either "Извините, а не подскажете как пройти к метро?" or "Извините, подскажите как пройти к метро, пожалуйста"? I mean, the first sentence ought to have "подсказать" in the second person plural, while the second sentence should have this verb in imperativ plural form (and not a question). Or am I missing something?
You are right. Unfortunately some native Russian speakers are not that good at spelling when it comes to unstressed vowels. They are pronounced roughly the same, so that's no help, and not everyone was paying attention at school where they explained all those grammar rules.
It may be true to some extent, but as a Polish speaker (Russian and polish have some similarities) we have a different word for the actual underground subway than for the restaurant. The word used above refers to the mean of transport only, as if we were asking for a restaurant we would say just its English name like we do in the non-English speaking countries