2) Assuming its best to plan for the 90 minute-ish layover, how is the station there for hanging around that long? I've never got on or off a train there before. Just passed through.
30th street is one of the most impressive stations in the Amtrak system. You can check out the station, check out the SEPTA boarding area on the upper tracks, plenty of places to get food. If you had more time, you could walk to the SEPTA Market-Frankford subway station and take the subway to see something of downtown Philly, but a 90 minute layover is way too short to attempt that. If you have never seen 30th Street, take the 90 minute plus layover and at least see the station.
You can't see much of downtown Philly from the underground subway.
However, if the idea is to get over to Center City and walk around to see the sites, then the correct answer is to avoid the subway where you have to pay. Instead, head upstairs at 30th Street to the SEPTA commuter rail and ride that 1 stop to Suburban Station and then walk around. It's free and its faster. Odds are that no conductor will ask for a ticket, but if they do you show your Amtrak ticket and that is considered as valid fare between 30th Street Station and the Market East Station. Suburban Station is in between the two.