There are masses held at the cathedral. You can check the schedule on their website. It's free to attend mass. As for your bag, not sure if it's allowed

Purchase tickets online to avoid the long lines. Go early in the morning so it's not too hot. Do the tower first because it's somewhat tiring if you're not use to doing stairs. Then spend the rest of the time in the cathedral itself. It's fair big so plan for at least 1 hour.

In Spanien werden kurze Hosen in Kirchen nicht so gerne gesehen - also besser lange Hosen tragen

there is a board outside the main entrance which posts updates of the masses each week. I think it posted in spanish and english but it is fairly easy to work out times and dates. good luck!

Hi. I purchased tickets from a tour group online.

Oui

En un lateral de la pròpia Catedral, estan las taquilles. Mi consejo es q las puedas comprar por Internet. Tambien mi consejo, de q puedas buscar un tour guiada "solo" de la Catedral. De esta forma, el tema de las entrades y accés, lo tendras solucionada. Por cierto, visita casi imprescindible, si estàs en Sevilla.

yes of course , they just ask you not using a flash...