Finding the subject in Japanese. 自分だったら、これを大きくなってから見たら感動するだろうな

Question

This is from the comments on this youtube page where an infant is cuddling with a bemused cat.

The commentator is writing on how cute the scene is. I am having trouble figuring out some of the subjects (which can be hard for students of japanese). If anyone would care to point out the subjects or correct my attempt at a very loose translation, it would be greatly appreciated.

I'm having the most trouble in figuring out the first line. (The numbers only indicate the lines):

1. 自分だったら、これを大きくなってから見たら感動するだろうな。
2. なんの気なしに一緒に暮らしている猫が
3. 昔から、こんなに愛情深く寄り添ってくれていたなんて。
4. 「猫さん」「猫センパイ」と呼びたくなるレベル。

1. To see these two growing up together would be so moving. (for me the speaker)
2. Look at this laid back cat just living together like its nothing,
3. snuggling up with so much love, must have been doing that for a long time. (for it to have been so comfortable)
4. At this level I'd like to call him Mr. Cat, or even Prof. Cat. (superior cat)

So in line 1, I'm having trouble understanding who this これを大きくなってから applies to. I'm guessing it's the kid and cat growing up together, not the writer growing up. Also in line 4, perhaps the writer is saying this:

"Mr. Cat, at this level I'd like to call you Prof. Cat ((superior cat))"

I know senpai doesn't relate to professor, but I just chose something someone might say in English. Also, does と呼びたくなる mean the speaker (him/herself) wants to call the cat senpai, or does the speaker want the cat to be called senpai by all in general? (First person versus third person plural.)

Answer 1

The key is the これを大きくなってから見る in the first line. The subject on both sides of the te-form is the same (自分).

1 If it (the baby) was me, I would surely be moved if I saw this after I grew up.
2 That this cat, who I had been living with without thinking anything much of it,
3 had been snuggling up affectionately to me from such a long time ago (/young age).
4 (It would be moving) To (such) a level that I would want to address him with "san" or "senpai" (treat him as an equal or as my senior).