Yes. Yes, for two reasons: allergic inflammation in the sinuses increases blood flow, so bacteria in the sinuses are better able to get the nutrients they need to grow. Second, sinus health is all about drainage - and the inflammation of allergies causes swelling that impairs drainage. Add the inflammation of a viral or bacterial infection, and the drain openings swell shut -> acute sinusitis.
Possibly. Your allegies may cause edema and swelling of the mucus membranes causing inflammed sinuses.