I was in Singapore earlier this month and took NH 844, the red eye from SIN to HND. As a NH Diamond, access to SilverKris Lounge's First Class section was granted and noted on my boarding pass (I was flying Business, but upgraded from Y.) Based on the verbiage on NH's website, Diamonds should be granted access regardless of boarding class, as it goes out of its way to clarify that Diamonds can access the SilverKris Lounge without having a First or Business ticket. Worth a call to customer service ahead of time to clarify, if you're worried.
The T2 lounge set-up is not as glamorous as T3. With no natural light, it feels a bit like a dungeon and outdated as well. It's actually been 8 years since I last visited the T2 SilverKris First Lounge, but I feel the food spread and customer service was significantly better before. The main buffet section was mainly Chinese food, but nothing memorable; lots of mushroom and chicken dishes, not premium ingredients (think glamorized Panda Express.) The made-to-order counter was only offering dim sum and one rice dish, which didn't look very appetizing. They also closed the counter around 9:00pm. In contrast, the ordinary SilverKris Lounge in Hong Kong, for example has 2 or 3 rice and noodle dishes made-to-order, with attendants proactively offering it to diners. In my 2.5 hours at the SilverKris First Lounge, I did not see one SQ staff roam the grounds, only a guy pushing around a trash cart, clearing plates and glasses - to be honest, not a classy way to pick up after others, as he was dressed similar to a janitor in a gym.
An unpublicized perk was that check-in was also available at Premier Check-In, operated by SATS. According to this Business Traveller
link, it has been available since 2010... was not aware!
ANA's check-in counter opens 3 hours prior to scheduled departure at SIN. I wonder if you could check-in earlier than that at Premier Check-In - Does anyone know?