Vocabulary doubts: 「眼の所が少し高くなっているが、［…］一文字に結んでいる。」

Question

I'm having vocabulary difficulties with this story about a frog hibernating in one's backyard:

ちらっと見たぐらいでは、おそらく誰も蛙とは気がつかないであろう。わずかに、眼の所が少し高くなっているが、その眼をこれ以上は閉じられないほど、しっかりと一文字に結んでいる。 ― ああ、冬眠なのだ ― とわたしは呟いた。

1. 一文字に結んでいる： Does this refer to eyes completely closed, eyelids forming a thin horizontal line (I guess so)? Or does it have to do with a frog's horizontally-shaped eyes, like this? (Same for later: 「泰然と目を真一文字にしている。」)
2. これ以上は閉じられないほど： "To the point that [the eyes] couldn't be closed any more than this"? Does this means that the eyes are completely closed, or that they are eyes that can only close up to a point?
3. 眼の所が： What's the role of の所 here? Is it "the area of the eyes", or just an abstract noun (形式名詞) thing?
4. 眼の所が少し高くなっているが： What does this mean? That the eyes are protuberant/salient? If so, why does it connect with adversative が to the next sentence?

Answer 1

1. 一文字に結んでいる
   As you guessed, completely closed. Not only supported by the context, but also by the verb 結ぶ, that means to make two things join each other, thus "shut closely". 一文字【いちもんじ】 is a depiction of a straight line so thin that barely has a width.

2. これ以上は閉じられないほど
   Depends on context, but here the eyelids are shut to the end, as a consequence of #1.

3. 眼の所が
   It is a 形式名詞, but maybe not quite what you thought. Basically you want to assume noun + ところ as "where noun is", thus in this case it's "where the eyes are" or "around the eyes".

4. 眼の所が少し高くなっているが
   You're right, it's "protuberant/salient", but が doesn't only means but. Its baseline is making "gap" or "shift" from the flow of previous clause (「もしもし、田中ですが、どちら様でしょうか？」). Here it seems to be serving for grammar rather than meaning, connecting the before and the after like a crooked relative construction. It's indeed used to translate English continuative relatives.