1: A quick way to find the numbers on the top is to add the two first numbers in the bottom row , so if the first two numbers are 3 and 4 it would add upto 7 which you put it on the first space in the second row. Now to finish the rest of the triangle all we need to do is take 4 and another number i.e 5 so 4+5 = 9 and that's the second gap. To find the number at the top all you need to do is to add the two numbers on the second row = 7+9 which is equal to 16.
2: Yes, if you change the bottom numbers the top numbers will obviously change as the sum of the numbers are different but the rule for how to find the top number will stay the same.
3: Yes, because if you follow the same rule it would not make much of a difference about the numbers.
4: The way to find out the numbers on the bottom with only the number on the top to consider is by to find two numbers that add upto the top number.For example if the top number is 34 take 20 and 14 and now find 3 numbers on the bottom by takinf two numbers which go into 20 that might be : 8 and 12 now for the third number. Since the second number on the row above is 14 and one number on the bottom row is 12 so 14-12 = 2. There you find a full triange completed with only the top number to consider.