No, exp reduction is how you get people to stop queuing because their team keeps losing fights. I've seen that happen in other MMOs with PvP battlegrounds. EXP should be the same regardless of who wins or loses if you want to incentivize participation.

I've seen very few people actually go AFK; rather, they run around base, so a timer really isn't helpful. EXP based on damage/healing means either people get shafted because they keep getting unlucky or have a bad team, or even worse, we'll see people start all playing the same classes based on DPS/HPS.

I don't know if there's really a good solution. Right now it's annoying, but there's enough push towards bases to keep it interesting even if they hang back. If the rewards for taking bases better contributed (or more overtly contributed) to the tally, then we'd see more pushes from other teams to take them, making 'afking' at base less of an option. And for those people who really do like defending, conversely, there could be an actual benefit to having people stay there (such as maintaining the occupation status).