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?
"Pardon" and "excuse" both hold the same definitions. They are synonymous. Both can and are used to interrupt, interject, gain someone's attention. Both are common. My only opinion lay in that I feel "pardon" is the more polite way to gain someone's attention, given the two options. Saying pardon in this way may seem non-standard to you, but it is not -- at least where I'm from.
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
I don't understand why "Sorry, where is the metro?" is wrong. It just tells me I've used the wrong word, but doesn't specify. To echo a previous, subway is a fast-food chain on the west coast - we don't really have underground trains and we literally have public transit called Metro. It's also not uncommon to initiate a question to a stranger with "sorry [i.e. pardon the interruption], where is ...?"