Many sources don’t mark all vowels with a breve or macron.
Sources that intend unmarked vowels to be understood as short will generally not use breves at all. Examples are Lingua Latina per se Illustrata and Wiktionary (the latter as a matter of general policy; some entries may go against that policy as it is a user-edited dictionary).
Sources that use macrons, breves, macron-breves and unmarked vowels usually don't intend to directly indicate the length of unmarked vowels. I'm not sure if the reasons are explicitly laid out somewhere, but usually this occurs in contexts where vowel length is either predictable, uncertain, or thought to be unimportant. There are two main contexts where this is especially common:
Vowels in the final syllable. The length of these vowels is in most cases predictable from the way that a word inflects or from its part of speech. Sometimes, the length of a final vowel is also uncertain or unimportant. For example, several large classes of words with final -ō have variant forms with final -ŏ; the distribution of these forms varies depending on the era you look at, but there is no difference in meaning between the long and short vowel variant. This variation with certain words ending in o is why Collatinus shows a macron and a breve on the last syllable of "ēdĕrō̆": it means that the vowel can be found as either short ŏ or long ō.
Here are some web pages with rules about the length of vowels in final syllables:
Vowels in a closed syllable (i.e. before most clusters of two or more consonants). These syllables are treated as heavy in the scansion of Latin poetry regardless of the actual length of the vowel itself, so it can be difficult to know the vowel length, and it is not necessary to know it to determine the correct scansion of a line. In a few special cases (e.g. vowels before ns or nf) the actual length of the vowel before a consonant cluster is predictable, but in most cases, it is not completely predictable.
Note: the Collatinus conjugation/declension tool seems to consistently use macrons to mark syllable weight rather than vowel length, so it won't tell you that the a in actus was long while the a in factus was short.
Diphthongs are not generally marked with either macrons or breves
“Diphthong” is not a very clear term, and I think you might be conflating a few different concepts.
The most consistent use of diphthong is to refer to something like ae, au or oe pronounced in one syllable. This type of diphthong starts with a vowel and ends in a "glide" (spelled e, i or u). The other diphthongs you mention (ei, eu, ou, ui) are very rare, and often coexist with alternative forms that do not contain a diphthong.
In my experience, traditional dictionary leave both letters of this type of diphthong unmarked with no macron or breve. Before a consonant, this kind of diphthong always creates a heavy syllable, the same as a long vowel like ā or ō or a sequence of a vowel and a coda consonant like ar, al, an.
Latin syllables could start with the approximant consonant sounds /w/ or /j/ (also called "semivowels" or "glides"), as in volo or iacit. A glide followed by a vowel is considered a diphthong in some languages, such as Spanish and Italian. However, while there may be some texts that refer to these sequences in Latin as "diphthongs", they are not usually called diphthongs in modern English descriptions of Latin. When the letter U/V or I/J represents an approximant, it is never marked with a macron or a breve. The vowel letter after a glide is marked the same way a vowel letter after a consonant would be marked (e.g. vŏlo/uŏlo, jăcĭo/iăcĭo).
Syllables starting with the consonants /k/, /g/ or /s/ could also have a glide between the consonant and the vowel; alternatively, you can analyze one or more of these combinations as a unitary sound (/kʷ/, /gʷ/, /sʷ/, spelled "qu", "gu", "su" respectively). The u in these syllables is not written with either a macron or breve.
A vowel letter before the glide /w/ is almost always marked long or short according to its actual length: e.g. căvus = /ˈka.wus/ with a short vowel, suāvis = /ˈswaː.wis/ with a short vowel. (Loans taken from Greek words with αυ or ευ may constitute a very insignificant category of exceptions where "āv" or "ēv" represents a diphthong, as in Agave /aˈgau.eː/(?).)
A vowel letter before the glide /j/ is often marked with a macron in words that probably actually had a short vowel followed by long or double /j.j/: e.g. maius or majus was most likely pronounced /ˈmaj.jus/ with a short vowel and a long approximant, and is found in some refernce works as māius. Lingua Latina per Se Illustrata doesn't generally do this, but Lewis, as well as Lewis and Short, does.
