Question 1 - Yes this can be done. The only error you will get from the leadwire itself is if the resistance of the leads are mis-matched. This can be avoided by using all one color wire that comes from the same spool. Also, using a 1000 ohm part versus a 100 ohm part helps to keep the overall leadwire resistance a lower percentage of total resistance. Finally, your part will be much less expensive if you allow the additional wire to be spliced on. Manufacturing with 300 meters of wire directly in the sensor would be very difficult.