No they can't. You are required to use the straight line method, 15 years, for qualified leasehold improvement property. However, in 2008, such property is eligible for the 50% bonus depreciation.

It is considered a leasehold improvement if it is affixed to the property and when you're installing a new unit.

It is if it is permanently installed, and you are leasing the property.

No, because Leasehold Improvements revert to the lessor at the expiration of the lease term and a sign does not "improve" the leased property.

Painting can be considered as repairs and maintenance as it is part of maintaining the property.