Velocity is when you throw an object horizontally.
No. Velocity is a vector, it is a measurement of the change in position w.r.t. time.
It applies vertically as well as horizontally.
The consequence of the throwing is, the object will go with relatively flat velocity.
No it won't.
It will follow a parabolic path.
You need some form of lift to keep it flat, unless you manage to throw it fast enough that it goes into an elliptical or circular or hyperbolic path.
If it is in air, the horizontal component will be reduced due to air resistance. The vertical component will increase (or become more negative) due to gravity, until it reaches terminal velocity.
Get a playing card and throw it, see what happens.
Unless you get it just right it slows down, a lot.
If the object has a machine with spinning blades horizontally, the velocity will always increase till a certain speed. That means an acceleration happens due to the machine.
Yes, where that speed is where the force due to air resistance is balanced by the thrust of the "machine"
This will apply for an aircraft as well.
It does.
That is why they don't keep accelerating like you claim.