This でな is indeed the te-form of だ, followed by な, a masculine sentence-final particle. A sentence-end で can have several different roles. Here, it may be a reason marker (i.e., explaining to someone why he has a sweet), in which case the combination of で + な roughly corresponds to "you know" in English. Or it may be a simple "continuation marker". As this ...


かんーわええのぅ。。 鼻水までもが愛おしく見える。世界いちかわいいよー。 They meant 「かわいいな。」/「かわいいなあ。」, "Wow that's cute." They wrote it in an emphatic, lengthened, and nonstandard (dialectal or classic) way. (な/なあ indicates 詠嘆/exclamation. Related post here.) (What they meant is also clear from the last part of their comment, 「かわいいよー。」) いい often changes to ええ in western dialects. ...


"oboe ga aru you ja no", means "you seem to remember." "no" in this phrase has no negative meaning.

Only top voted, non community-wiki answers of a minimum length are eligible