The arch is broken and mathematically should fall down.
This a premiss of the show.
Something is playing sillybuggers with gravity.
That sounds like an engine to me.
Just because all the ships had sublight engines it doesn't mean that they all had ftl engines, because there could have been just one ship that generated a gate... Even though that they spent hundreds of years in stasis, it sounds more like that they used time dilation approaching light speed to get to earth in hundreds of years instead of thousands. Even though they might have spent hundreds of years accelerating and deceleration and the bulk of their journey happened in a matter of seconds.
Data's Mom doesn't want to share or hand over the technology... Which probably means that it's massive if she doesn't have plumbers running around making sure her evil plan works out secretly in the dead of night.