Of course, if the ship is one kilometer long and the breach would envelop the ship in six hours, the breach should have been noticeably moving.
If it were moving at a steady rate of speed, for the explosive force to reach the bow in six hours, the "cloud" should have been expanding at just under 20 millimetres a second (their prospective) or about 1.2 metres per minute. they were in engineering for a couple of minutes.
But that assumes a steady expansion. You need to take into consideration that there was (at that time) a relatively small opening into the warp core. The main explosion wouldn't occur until the body of the warp core housing was torn apart. What we (and they) were likely seeing was a initial pinhole venting plasma.
The Enterprise D is 642 metres long.
The warp core is approximately two-thirds back from the bow.
Or approximately 427,572 millimetres.
There are 21,600 seconds in 6 hours.
19.795 millimetres per second expansion.
Second, in space the ship weighs nothing.
You are completely correct. While a ship in space has mass, a ship in space has no weight.
Again, you are correct.
It's like saying that Neil Armstrong could've picked up the moon and tossed it out of orbit if he wanted to. I mean, it "weighs nothing", right?
It's the Moon's mass being attracted (mutual attraction) to the Earth's mass that holds it in orbit. The late Neil Armstrong (despite his last name) would lack the strength to move that much mass.