One thing I feel confident in saying is this: The writer of Revelation clearly wanted to convey to the reader that the great crowd is in heaven.
Think about it: Why would reference be made to a great crowd of gentiles standing before the throne and serving God in his temple? In Judaism, gentiles were forbidden from entering the temple. It was considered a sin for a Jew to even enter a gentile's house, far less the temple! Early christians were just getting used to the idea of gentiles mixing with Jews in worship under the new arrangement of christianity. Undoubtedly there could have still been some few lingering pockets of discrimination or apprehension about gentiles worshipping with Jews. Some gentiles may have still felt a bit unworthy given their non-Jewish background. This is the context within which one has to read Revelation chapter 7's statement of a great crowd of gentiles serving God in his temple.
It is my belief that such a passage was meant to silence any lingering doubts in the minds of any Jewish christians about gentiles truly having the same hope as they had. It would have also served to reassure any gentiles having niggling deep seated doubts about their worthiness. Revelation's stating that a gentile group is serving God in his temple is clearly meant to cement the idea of the worthiness of gentiles to enter the intimate congregation of God - into his very temple! I believe this is the whole purpose of mentioning the great crowd from every nation right after mentioning the 144,000 Jews.
I believe the 144,000 are literal Jews whether or not the number is symbolic. It's a way of showing that God's blessing on the twelve tribes of Israel will in fact have a true lasting fulfillment in a remnant of faithful Jewish christians. So both God's loyalty to faithful Abraham and his impartiality to people of all nations is being highlighted. Revelation 7:4-17 is John 10:16 (Other sheep = non-Jewish sheep) and Ephesians 2:11-19 in symbol.
It is also possible that the great crowd and the 144,000 are one and the same! Perhaps the 144,000 are described as Jews because it refers to all christians who make up spiritual Israel. But notice something: John doesn't see the 144,000 in that vision - he only hears the number that are sealed. Then he sees a great crowd of every nation. Do you see what the writer could have been doing? The writer could have been employing a kind of surprise literary device. John in vision hears as if a limited number of literal Jews are being sealed only to have the lights turned on, so to speak and, surprize! surprize! It's actually a great crowd that no man can number and people of all nations are included!
The Watchtower interpretation of "before the throne" doesn't fit the context. Saying before the throne means in the sight of God but still on earth makes no sense when you consider whythey are "before the throne". John is told that they are before the throne because they washed their robes in the blood of the lamb. Clearly, being "before the throne" is being presented as a great privilege that can only come from washing your robe in the blood of the lamb. But how can it be a privilege to just be in the sight of God on earth? Aren't even the wicked who have not washed their robes in the blood of the lamb also in the sight of God on earth? Wouldn't the wicked also be "before God's throne" according to Watchtower's twisted interpretation? This is why they must be literally before his throne - in heaven. Also, if they are literally on Earth as they have always been, why does the angel ask John "where have they come from?" "Come from?". Wouldn't they be where they always have been if they're on Earth? Watchtower's interpretation doesn't make much sense.