There is no rule on how to place stress in combinations of vowels. It is the other way around. Any two vowels can be stressed in any way and, depending on whether they are both in the same syllable, and whether they are strong or weak, they either form diphthongs or they don't.
In your examples the vowels are divided into syllables like this:
+ Causa = Cau-sa.
+ Veamos = Ve-a-mos
So in causa the two vowels are in the same syllable and in veamos they are in different ones.
The combination "au" is considered a diphthong, as you already found, and the combination "ae" is not, as they are both strong vowels.
As for the second question: As I said before, any vowel could be the stressed one, so the reference says that in case of two weak vowels together if the stress is on the first one then they are pronounced as two separated syllables. And if they are in separated syllables then there is no diphthong. Again my point is that it is not that the combination of syllables tells you how to pronounce the word, but the pronunciation tells you if it is diptongo o hiato or neither.
After trying to look for other examples I think the fluido example shows that perhaps the reference is wrong and it should say second instead of first.
The syllable split for fluido in the reference is fine but the stress is on the i (flu-I-do) because if the stress were on the u then the word would be considered esdrújula for having the stress on the third syllable (counting backwards) and then it would be written flúido. The main thing is that the pronunciation would put the two vowels in different syllables so there would be no diphthong there.
An example of a diphthong with ui combination is buitre (bui-tre) where both vowels are pronounced as one syllable.
I think you got confused thinking that this is a rule on how to pronounce a combination of vowels but for me diphthongs are not ways to help you pronounce words. The way to know how to pronounce a word is a combination of knowing this plus watching if the word has a "acute accent" (tilde á é í ó ú) and knowing when these tildes are used.