Quando siamo andati noi non c'era nessuno, siamo entrati subito.

Yes, take line 13 to the Basilica stop. There is a museum about the history of Saint-Denis and a large park near the Basilica.

Previous answers are probably the best with taxis being quicker. However the Metro is also available and a one one trip would take about an hour. The Basilica is quite stunning and well worth the visit. I am not sure if you have 7 FULL hours available to you. Timing could depend on local or international flight requirements and how long it takes to get through Immigration and ticketing.

no, just normal clothes covering your body ;:) no hot pants or swim suits and men must take their hat off (in every church, I think)

Additionally, the crypt is part of the church, so you just walk down into it. There is no need to make an extra trip. I would recommend not driving, but taking the metro to the church. Take Line 13 in the direction of Saint-Denis-Universite. Get off at "Basilique de St-Denis". Signs will easily guide you and it is literally less than a five minute walk from the station to the church.

Si a noi nessuno ha detto nulla...

We were dressed casually on the way to rugby so bot formal, however as said respectful as it is a place of worship. The entrance is free but you have to pay to enter the area where the royalty is buried.

Judging by the shop signs St Denis seems to have many Turkish shops and there are certainly many more "foreigners" evident here than in the more touristed areas of Paris, but the Cathedral is only about 150 metres from the Metro across an open square and adjacent to the Town Hall so I would feel very confident about going there as a single woman. If you want to venture further away from the Metro, the main shopping streets were very crowded so once again there would seem little problem - at least during the day.

Sorry I don't know, but try searching online as Maddie suggests.