1.How would interstellar spaceship without FTL or wrap capability measures instantaneous speed accurately?
Other answers implied but no one blatantly stated the answer: you CANNOT measure an absolute velocity, which becomes especially obvious when in outer space. As far as we can tell, there is no such thing. Even on Earth, your "instantaneous speed" is actually relative to... the surface of the Earth! (Note that by definition, "speed" is independent of direction, and for a ship moving in 3-dimensions, the directional vector of its velocity is vital. Shades of Wrath of Khan!)
Unless the exact timing is important, such as plotting out an intricate "battlefield" or going (fairly slowly) between planets inside a stellar system, you can usually assume that your planets aren't moving. Only the relative speed between the stars might be of any interest at all, and rarely that would be either. Relative position in 3-d space might be of interest, since there could be a considerable Z-axis distance between 2 stars that have similar X & Y coordinates.
You didn't ask, but fortunately, several good suggestions about how to measure relative velocity were suggested.
In practice, you would probably use a combination of things to describe your velocity, depending on your technology, location, and relative velocity; (continued in explanation for #2).
2.If instantaneous speed is useless for space travel then what kind of measurement would be adopted instead? (e.g. light year is used instead
of miles or kilometer etc.)
That will depend on how "fast" you are going relative to other bodies in space (that you care about), i.e. your "delta v(elocity)". Some examples could be:
- small velocities/small scale objects - meters/second up to km/h
-- typically for small boats, space-walkers and small to mid-sized ships approaching airlocks, space stations or other ships, missiles
- inside stellar system cruising - likely in km/h up to km/s, which could get very large depending on your thrust technology and travel time. You would choose between /h or /s depending on how long it is traveling or how big a number you want to make it look. For example, the New Horizon probe to Pluto (fastest ship to date) left Earth at 58,000 kph, which is about 16 km/s.
- interstellar cruising - could be the same as inside the stellar system, but probably will want it to be expressed in km/s because you're going to want to go way faster than even New Horizons. Hopefully you have some kind of continuous acceleration, and in that case, you'll be moving (relative from your departure place) a lot faster than even New Horizons.
- lightyears - without an advanced, non-Newtonian (i.e. "impossible") space drive, you will never need to measure velocity in light-years unless you're trying to be cute. e.g. "lightyears/century"
- Red/blue-shift or C - Mid-way through your decades/centuries long trips, you may find it convenient to use the red/blue-shift percentage of a set of "stellar beacons" or even your destination star, since your relative velocity could be a significant fraction of light speed... if you have a significant amount of acceleration the whole time. Or just use fractions or percentage of C (light speed) to simplify and make it more understandable to most people. E.g. 93% C or 0.93 C
- Distances could run the gamete from meters to kms to thousands or millions of kms to light-years (remember that light-year has nothing to do with measuring time, it's just the distance unit of how far light travels in one year, the same as a meter is how far light travels in a fraction of a millisecond)
You would certainly want to measure distance between stars in light-years. It used to be common to use parsecs, but that's very Sol-centric.
Acceleration - likely in meters per second squared for known technologies, or as is commonly expressed, maybe in "gravities" where 1 G = 9.8 m/s/s - the acceleration due to gravity at sea level on the equator on Earth. This would only be needed if your ships could accelerate very quickly though. Unprotected humans can only stand a few G's, maybe as much as 6 G's without injury.