If the Doctor spoke his name then they would know that it would be safe to come through, but all this time they must have grown more TARDISes or could have shared them and as soon as they come through they all escape to different time periods and places, Gallifrey would be completely abandoned so there wouldn't be anyone to be killed and the Daleks would all leave as soon as they realise there's no point attacking the place. Since no other race has mastered time travel like they have, they can all agree to rendezvous at a time when the Daleks are extinct (like around the year 100 trillion) and plan to rebuild their civilisation (or make the final sanction at the end of the universe, there's nothing alive anyway). The citizens of Christmas could also be saved as the Doctor can fit them all into his TARDIS.
The purpose of the Time Lords' message was two-fold:
- To confirm for The Doctor that they were still alive (which he was probably unsure of after "Day of the Doctor"
- To determine if it was safe for them to come back through.
They knew they were sealed away in the pocket dimension at the height of the Time War, when the entire universe was at risk of being destroyed. Presumably, they haven't been able to watch what was going on since then, until they found one of the cracks and could leak a message through.
By refusing to say his name at Trenzalore, The Doctor was telling them that it was not safe to come back. He knew their reemergence would just kick-start the Time War all over again, so he never gave them the signal. However, Clara speaking to them would have clued them in that The Doctor was there. They would have been smart enough to add those clues up and realize that The Doctor was warning them to stay away a while longer.
Also, it's completely untrue to claim that no other race has mastered Time Travel. The Daleks are at least as good at it as the Time Lords -- that's how they managed to wage a Time War in the first place. In addition, a number of other time-aware races exist, and within a few centuries even humans had figured out Time Travel (e.g. the Time Agents little wrist thingees). So, there really was nowhere and nowhen the Time Lords could go that the Daleks couldn't chase them.
Because Gallifrey was weak, and ready to fall.
The Timelords were sealed away at the lowest point they had. There was no further last redoubt. Final defenses had failed. The Daleks had won. Period.
While we dont know what relative time has passed inside Gallifrey's pocket, we do know that Timelord tech doesn't always react well to being away from the universe. They could well lack the ability to rebuild their power while sealed off. The biggest proof of this is Trenzalore itself - if they were strong enough to hold off their enemies, they wouldn't be asking if it was safe to come out.
Leaving Gallifrey behind isn't much of an option either. Many of their facilities to do things like create TARDISes would be planetbound. Their civilians would not be easily evacuated. Daleks have time travel tech, so they would be pursued.
They needed to come back to a point where they would have a chance to rebuild. They didn't get it at Trenzalore. If the Master is loose, maybe they found somewhere else.