As a former professional librarian, I love technical discussions like this! I have struggled many times this problem, especially when the library holds only a few scattered issues of a periodical or (as very often happens), the publisher itself makes a numbering error.
Normally a volume consists of all of the issues published in a year, with the issues numbered sequentially therein; a volume can have from just 1 to 360+ issues (some daily newspapers used this numbering scheme). With irregular frequency publications, volumes may not be annual, but they are always sequential (e.g. Volume 1 is from 2015, volume 2 is from 2018 with nothing having been published in between). I have only ever seen whole numbers (numbered consecutively from Issue 1 to end of publication) with video game magazines.
My only contribution concerns the dating of issues. Can we learn anything from the release dates of the games covered in the magazine? I once encountered a British game magazine with no dates listed anywhere (cover, masthead, copyright). Almost all of the games reviewed had been released within a few months of each other, however. Thus I was able to make a best guess as to when that issue was published.