Flat Earth is travelling in a straight direction.
I would say "mainly straight direction" - we don't know if it's actually completely straight or if there is some variation. Some experiments such as pendulum experiments suggest there that is some other directional motion involved.
Can it tip over? Why/Why not?
We don't know. I would guess not because the energy pushing us forward is behind us, like if you blow up a balloon and let go of it, while it flies in lots of different directions - it has to keep generally facing the same direction relative to the propulsion and the direction.
Can it decrease/increase speed? Why/Why not?
Again, we don't know. I would say that there are two possible outcomes. Either it will continue on at the same speed for ever, or it will slowly slow down. Maybe it is already slowing down, but a t such a rate that it is unrecordable. Of course it's also possible that rather than a linear deceleration, it will just suddenly stop. That would obviously be apocalyptic for the world as we'd suddenly experience 0 gravity. No doubt the socialists will blame capitalism for this occurrence (as they should).
Can it change direction? Why/Why not?
We don't know. Obviously it hasn't changed much as far as we know, but it could in the future. It's hard to see a reason why it would change significantly. Perhaps the propulsion is not symmetrical or perhaps we could fly through a certain higher density substance that would slow parts of it down. This is merely speculation. I think the answer would probably be that it could change direction for those reasons, but we can't be sure until it happens.
And explain how seismic activity have, or don't have, any effect on these elements.
I would guess that it would have no activity although obviously we can't tell for sure. It's just my impression that seismic activity is so small, relative to the entire Earth, that it wouldn't have any significant effect.