I just now read the tips and notes for numbers again. Only now I understand the meaning of the following:"...either keep the counted entity in the genitive (Nom. or Acc. of the whole phrase) or match it to the case of the whole phrase (otherwise)." In some way the exception is nominative and accusative, which need always genitive.
Someday they probably will but until that I suggest reading some of the resources you can easily find on the internet like http://cokdybysme.net/pdfs/locative.pdf https://en.wikipedia.org/wiki/Locative_case#Czech http://utkl.ff.cuni.cz/~rosen/public/GGG/Czech_essent_grammar.pdf
Certainly not just j, Czech only allows l and r between consonants. And even for j+(a wovel other than e) it sounds alien (remember that bě,pě,vě is pronounced bje, pje, vje). That would ve found in certain other Slavic languages. Compare
řeka, rijeka, река
Russian е is pronounced je, like Czech ě. я is ja and ю is ju.