https://www.dlsite.com/books/work/=/product_id/BJ142475.html
https://book.dmm.co.jp/detail/b064bcmcm01044/

translation info:
raw: /s/9e221e4484/1043722-166
translation/typesetting: biribiri https://biribiri.mangadex.org/
commissioner: espeon https://gender.tf

read early and support more gb at https://gender.tf !

I'd probably end up beating up my mirror self because she would definitely be ugly.

Base +24, the_king901 +7, reTs98rf +4, CorruptedNerd669 +7, Mahos +13, bvvbn +7, Cmdrtimbt +6, Shupuu +6, FeelTD +8, Hiltrax +7, sukto +6, hazari_13 +6, flere101 +6, and 9 more...

Hm, what happened exactly? Did they merge or did she possess him?

Base +6, asdfghfd +7, phantominferno +6, simplesimon32 +36, IncognitoMaud +6, Soldier A +6, imanoterone +4

They merged. The thoughts on the last page seem to be first from the boy then from the girl. Either he was "in control" at first when awoke or just a bit of groggy confusion after the merge + waking up

Plus the whole "melt together" thing and "his" face at the end being feminine and "her" wearing glasses at the end despite his reflection(her) not having them before.

Although depending on how you interpret this there might not be much difference between possession and merging.

Base +6, Kraftz +6, Sylvrwolflol +7, bggm +29, Kiranta +6, throwaway2112 +6, MyraLune +7, TheKrister2 +6, hazari_13 +6, randomname12131412 +7, simplesimon32 +36, IncognitoMaud +6, Soldier A +6, and 4 more...

I'm surprised this isn't tagged as shota, the source is
/g/1361357/9f7e09140e/
/s/ee3e6ba346/1361357-165

Base +6

My own reflection! Now neither of us will have to be virgins!

Base +6, Phonoi +7, Naxuro +6, Temperature Critical +26

To me, the bottom of page 19 seemed to imply that she jumped into his glasses since she seemed to be able to transfer between reflective surfaces (she started off in the compact mirror), so my vote is more on possession, but honestly I would believe either one.

Base +5, imanoterone +4