With the understanding that I, or so I was told by literally half of the Japanese people I spoke to, speak Japanese with an Osaka accent:
AI- Pronounced as “eye,” the body part. One syllable.
EI- Pronounced as “ay,” as in “day” and “pay” One syllable.
Final N- Doesn’t add a syllable. It’s tied to the vowell sound before it.
Double Consonant- I really don’t know why these exist. Some people have told me they’re stressed syllables, but I’ve really never heard them stressed in conversational Japanese, even in Japan. If you break the word up syllable by syllable (Nip pon might have two P sounds, but Nippon doesn’t, I mean), the letter may be pronounced in both, but I’ve never heard that done, ever. They do not add any syllables.
In your examples:
“Nippon” is two syllables: “Nip” and “pon.”
“Hokkaido” is three syllables: “Hok” “kai” “do.”
Pronunciation Rule you absolutely need to know when it comes to syllables:
At the end of words, the letter U is almost always silent. “Kon Yaku Suru” (which is something aobut being engaged I learned back when I went to Japan to cheat a bridal place into giving me free cake samples) is correctly pronounced “Kon Yak Sur.” The incredibly common word “desu” is simply pronounced “des” (“Dress” without the “r”). This, obviously, cuts out a good number of syllables. The only exceptions I’m aware of is “bifu” which is said “beef-oo” and “biru” which is said “bee-roo.” I don’t know if this is a pronunciation rule on the English-import-words or a matter of incredibly common mispronunciation (the way Micheivous is often pronounced as mis-chee-vee-ous in English, though there is no second long E sound). Katsu’s U is pronounced in some “lower class” accents, but it’s doubtful this would be done in poetry.
Rule I see used in Japanese Haiku that may help you:
If you’re familiar with older English poetry, you’ve probably seen “cover’d” or such words with an apostrophe replacing the “e” in -ed endings, to cut down syllables. The same is often done in Japanese Haiku (though elitist pedants will snub their noses at you for cheating, more likely than not, as they do for the E thing in English) with the letter “R.” R is often very under-enunciated in Japanese, so, poets sometimes leave it out entirely to get syllables to fit.