How is performance downgraded if the windshield is damaged in flight?
The biggest restriction will be the reduced flight speed and cabin pressure which need to be observed with a damaged windshield. No pressurization will restrict the flight altitude to 10,000 ft or less. The crack will not increase drag, but force the pilot to continue flight at a less efficient speed and altitude.

Why high altitude is good for engine efficiency can be found here.

How to find the most efficient flight speed can be read here.

If pieces break off, the aircraft should not continue the flight, but land immediately.

To answer the other questions: Yes, sure, tail-mounted engines are known for ingesting stuff which has broken off from the fuselage. Ice from a leaking lavatory has caused engine damage and the imbalance made the engine shake so hard that it fell off. The whole windshield or substantial fractions of it could have this effect, but I doubt that flakes will damage the engine.

The aerodynamic pressure at the windshields tends to press them into the fuselage. Suction only starts above of the windshield with the curvature of the fuselage contour. However, with cabin pressurization added, the windshield is indeed forced out if it fails. One accident which was caused by using the wrong screws to secure the windshield is BA Flight 5390. The captain was sucked out of the opening left by the window and only his ankles, which got stuck in the flight controls, prevented him from falling out entirely.