If flying NW/DL, you will be in BE (former WBC on NW), not FC, although the NRT-SIN food service isn't really WBC/BE but probably more like the current FC to HNL and the hard product is the NW standard international Airbus 330 with WBC seats and good entertainment systems. You are considered to be flying international business class on both segments, not first class, so you will probably be unable to access any lounge that is reserved for FC passengers. (For example, at CDG, PE/PM, AMEX Plat, and international BE boarding pass do not get one into the dedicated AF FC lounge--it is reserved for only AF passengers flying their longhaul FC.) The two Skyclubs (both former NW Worldclubs) at NRT are conveniently located and wonderful. I'm not a beer fan, but don't miss the beer machines.

Your arrival gate doesn't matter much. All NW/DL operations are in the same area of Terminal 1 (gate numbers something like 14 through 30). Do not even think of trying to get to Terminal 2; this requires a boarding pass for a Terminal 2 flight and a long wait for the shuttle bus for your airline. It's even a very, very long walk to the areas of Terminal 1 served by other carriers. When your flight arrives, you follow the transfer/transit signs, usually staying on the upper level but sometimes just taking a long walk; you are segregated from departing passengers at this stage. Then you must clear security (and usually go downstairs or alternatively make a U turn) before you can access the departure gates and the lounges as well as a few shops in this area of the terminal.

Both NW/DL lounges are attractive, spacious, and very nice; I usually just go to the one that is closest to my departure gate. In the past, there always was a nice variety of food (rather substantial snacks including sushi, noodles, soup, small sandwiches, smoked slamon, etc. all labeled in English) and beverages (coffee/tea, soft drinks, wine, beer, limited variety of hard alcohol). However, since the downgrading of offerings at the old NW clubs, I would expect the food to be at the level of what is available at the old BE lounge at ATL when the international flights depart. They have a selection of international newspapers and lots of copies of a few glossy aspirational luxury-lifestyle magazines. There is a collection of white Apple computers with free internet access, I assume reasonably fast but I haven't tested it recently, and a variety of seating options. Cubicle-like work space is rare in the NRT clubs. Some seats have nice views. The restrooms (at least the ones that I'm entitled to use) have Western toilets and are clearly better than those for the public in the terminal. Except for a very few special dedicated FC-type lounges in places like FRA, HKG, CDG, SIN and LHR (I'm sure you know the airlines that have made such investments) these two are as good as you'll find anywhere.

Note (OT but important info for your trip), however, that unfortunately the lounge in SIN may exist no more. Nothing is now listed on the DL SC website to which the NW WC website re-directs. It was a contract lounge and nothing much--champagne and little bags of potato chips, as I recall, surely coffee/tea/water/beer too--a single small and depressing windowless room but better that waiting in the even more depressing atmosphere of the gate in Terminal 1. (Further back, I think there was once a small NW WC near the rooftop garden.) However, there are multiple Priority Pass lounges at SIN as well as some lounges where one can purchase one-time admission (compare the features before paying), open 24 hours in every terminal, so this shouldn't be a huge problem. Once passengers are through passport control (which requires seeing/talking with a person in the NW/DL check-in counter area after they have opened for the flight, even if one has already checked in for the flight and has no luggage to check), one can access all of SIN's airside facilities in all three terminals (but not the old charter/budget carriers terminal). There lots of shops (but most are closed in the middle of the night), free movies, a tropical garden, a swimming pool, sleep areas, restaurants, free internet-enabled computers, etc., in addition to a variety of lounges. It will be interesting to see if NW/DL will nevetheless arrange to grant access to some contract lounge for BE/WBC or PM/PE passengers. (Pre-merger, NW made lounges available frequently where there weren't supposed to be any lounges...for example, at SVO, they handed out vouchers for the Irish pub across from the gate before it was possible to use the Aeroflot lounge.)