1. Big mirror :)

2. If you have direct sunlight on the plants, that is bright enough. Try for as many hours as possible each day. If you don't have direct sunlight then your eye is very poor at judging the brightness. Either judge by how well the plants are growing or get a light meter of some kind. You can get a very crude estimate by seeing if your camera wants to use the flash to take pictures.

You might want to reconsider on the 'no roof holes' thing.

A skylight would help. There's a style I've seen with mirrorized mylar on the inner sides that captures light and bounces it all over.

Obvious question - do you have outdoor trees, shrubs, etc. that are shading the windows that you could trim or remove? Are there window treatments you can remove? Other than that, you won't get more light without structural changes or growlights, sorry!

In the place of a big mirror, consider using a big sheet of aluminum foil BEHIND the plants to reflect the sunlight you do get back onto the plants.