like I said, stefilmon -> fury mode -> rasenmon. both dont get listed, otherwise youd have agumon listing greymon, metalgreymon, wargryemon, and omnimon.
fury/rasen isnt a slide because we havent seen them switching between forms. fury just turns into regular - so thats digivoltion. if fury turned into rasen, then back to fury, then back to rasen etc then itd be slide.
also you arent even sourcing the correct chapters in the first place.
stefilmon -> rasenmon is chapter 21 and stefilmon to fury mode is chapter 22. you listed them both a chapter 21, but that is not the case.
so yes, only fury mode should be listed for stefilmon due to warp break, whilst fury/regular isnt a slide as they didnt mode change multiple changes between those forms. an example would be shinegreymon burst mode, thats not a slide because shinegreymon doesnt casually switch between regular and burst whenever he feels like it.
the reference book states the regular version is the "true form" (ie, the digivolved form) of fury mode and that it has to be fury mode first before it becomes the normal one. so ye, stefilmon -> rasenmon is technically a warp digivolution as it skips over fury mode.
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omnimon merciful mode has yet to be "ultra" in an appearance. hes not mentioned as a level in tri (likely mega), and is a mega in links, as ultra doesnt exist. if hes in the next console game, hell prob be ultra, but until then he;s a mega