Honestly, you're probably OK. The trace length is short enough that it's really not too significant, the important part is the balun and transceiver IC output impedance, which is probably only somewhat close to 100 Ohms differential. You may lose some range, but the fact that you're using a chip antenna is probably more significant.
Putting footprints in to play with the match might help, but it might also open up a big can of worms as well.
Yes, you should assume that the receiver is a 75 ohm load, with one conductor grounded (i.e. unbalanced).
Higher frequency signals will create reflections if they try to go through a point where the impedance does not match. A system where the source, load, and cables all have matching impedances will not produce any reflections (and it will also have maximum power transfer).
For sources, the source impedance is similar to a resistor in series with the source. For loads, it's similar to the resistor connecting the two conductors.
For cables, the concept is more complicated. Wikipedia has a good overview, but the rough idea is that the cable's impedance is the ratio of voltage to current in the cable. Common cable impedances are 50, 75, 150, or 300 ohm.
The antenna design software you use is designed for both transmitting and receiving signals, but transmitting is often more important, so they call it the source impedance. But, the antenna being a symmetrical device (from the transmitting/receiving point of view), it is equivalently the load impedance.
Generally, designers will provide purely real source and load impedances, so you don't need to worry about the imaginary parts of the source/load impedance. Note that the impedance of the antenna is a function of frequency, and your antenna will not be able to have a purely real impedance at all frequencies. Usually a Smith char (polar plot of impedance) is used to visualize impedances. If your impedance is wrong, then a transformer (either coils of wire or quarter-wave lines) could be use to match it to the 75 ohm load.
Best Answer
Trying to think what 433 is used for off the top of my head :) Is that the weak signals band?
At any rate, most 2-way radios are made to match up to a 50ohm antenna and the matching is left up to you. You can get an antenna that is already tuned, or you can do impedence matching through a number of techniques (see the referenced article below).
With a good match, you reduce standing waves. Standing waves build up when the radio sends out a nicely modulated signal but the antenna isn't resonating at that frequency and causes standing waves, which feed right back into the radio and can blow out the final stage.
The higher the output power, the more this becomes important. At very low power, say <1watt, the worst you have to worry about is the antenna not resonating and your signal not going anywhere. At higher powers, say 50+ watts, you can damage your transmitter in less than 1 second. Modern radios have built-in SWR detectors that will cut the power if it detects a problem. Those aren't always guaranteed to work though.
This page lays out a few key things verey nicely