Has there been any instance where a national record in any sport broke the world record?

Yes this is present in shooting where there is a list of National records that surpasses the world records.
this is because of the very strict regulations followed by "International Shooting Sports Federation"(ISSF). They only consider those events for world records which are supervised by the ISSF(like Olympics, World Championships, World cups, Continental Championships, etc). On the other hand, most member federations have more relaxed rules for their national records. This creates many more opportunities for record-breaking, especially so in the non-Olympic events, where World Cups are not held.

for the list of such events and the details of the National and World records have a look on the link below.

List of national shooting records surpassing the world records

and still it says that the list is incomplete, so I guess there are more out there.

Is 5'3 short for a girl?

In order to answer this scientifically we need to consult the growth chart for the country you are in.As I dont know where you are, lets have a look at the UK one here (second page, bottom right):https://www.rcpch.ac.uk/system/f...5′3 1/2 is 161 cm.Not surprisingly the answer depends on the age. If your

Why can't I pull ten chin-ups in a row even after a lot of training? What is the right technique for doing chin-ups?

Listen brother it doesn't matter how many pull ups or chin ups you able to do.Doing 20 to 30 pull-ups or chin ups in a row won't give you better result than doing 6–7 Chin ups with proper form and slow tempo.More push ups means you have more body strength ratio with your upper body to

Do high reps increase muscle mass or decrease muscle mass? Shouldn't they decrease muscle mass since high reps activate the slow-twitch muscle fibers?

You're completely correct. Low reps (6-12 reps) with high weights force your body to use it's ATP stores for energy use, as well as type IIB muscle fibers (which are designed for short and powerful explosions of energy, which lead to a growth in power and size). If you perform any physically-exhausting