つぷ (or つぷり, つぷっと) is not a new or rare onomatopoeia. You can google it (results may contain materials that are NSFW). It's basically a lighter variant of ずぶ (or ずぶり, ずぶずぶ, ずぶっと).

Only a few creative writers make totally new onomatopoeias in their works, and there is no "rule" for such creative activities. Some can be understood as variations of existing ones (e.g., [わきゃわきゃ][1]), but some doesn't even represent sounds. [This page][2] introduces some unique onomatopoeias based on their etymology (of course it includes めるめる).

Rules around commas are generally fairly loose in Japanese, and it's not uncommon to insert a comma before と. Here, that comma may help the onomatopoeia stand out from its surroundings.

  [1]: https://japanese.stackexchange.com/q/42931/5010
  [2]: https://sp.comics.mecha.cc/free/mechamaga/articles/onomatopoeia-manga