食べる時間＿＿ない: Is さえ/も/ででも possible in this context? (Please read further)

Question

*Please be advised that the following is from a practice book, with minimal altering. I sincerely hope this sort of question does not disagree with anyone here.

The question in the book:

忙しくて食べる時間＿＿ないのに、遊びに行けるわけがないぞ！
Relevant answers―
A.) でも
B.) さえ

Below is how I see the problem.

First of all, according to this question, I believe it is safe to replace it with も here, although it is evidently too weak a statement.

Secondly, let's take a look at でも:

a.) If で is a variant of だ, 食べる時間でもない can literally be translated to "is not (the) time to eat either." Thus, out of the question.

b.) What if でも functions as "even" as seen in this question, defined as "で 'in (the case of)' + も 'even, too'" by cauliflower cloudさん? This is where things start to fall apart for me. Is this simply wrong, or just unfit due to context and/or naturalness?

c.) What about で (as seen in a. above) + でも (as seen in b. above)? Although I assume it is unnatural, would it be correct grammatically speaking?

What's worse, the book provides these sample sentences:

1. 俺は今日吸う *タバコさえ* ないんだ… ( = タバコも)  
2. そんな簡単な問題、 *犬でさえ* わかるよ！ ( = 犬でも)

I do not understand at all how in sample sentence 2, でも becomes possible again. Does it have anything to do with what the subject in the sentence is?

Here's an incidental example from a Japanese-teaching website (Wasabi). Again, でも seems perfectly fine to me, but is deemed incorrect:

私の日本人の彼氏は「I love you」とでも言ってくれない。

The author of the article where the above sentence is from explains as follows, which regrettably makes no sense to me:

It’s wrong because it doesn’t imply any reasonableness.

Are all these instances a matter of grammar, or do they have more to do with context and suitable usage?

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*Trivia:
I apologize if I seem to be spamming this forum with questions lately. It's just that I've been stuck on this particular grammar problem for far too long. I've no choice but to resort to posting a new question here. There are several articles on the Net which deal with さえ and でも, but I honestly have not found one which hits the spot for my dull brain. I'd really appreciate it if someone would have another go at it for me. Thank you.

Answer 1

ugh... where to start... First, from that Wasabi website, it's clear that the writer contributing to that section does not speak native level English, which is going to cause you some natural confusion. Second, these words expressing the concept of "even" vary slightly in BOTH nuance and in strength. In your first question about "Too busy to even eat", も is definitely not strong enough to express that "even". It's too weak. も is literally nothing more than a marker of addition/inclusion. If you use it there, you will be understood, but it will come across as "I'm busy and I can't eat", which has no sense of "urgency" or "frustration" provided somewhat by でも and even more so by さえ. You can safely set aside your "a.)" example since it is clear from the examples on the website and the question from your textbook, that we are not meant to regard で as a separate particle. We're specifically being asked to evaluate the appropriateness of "でもない" vs "さえない" ... I understand, but do not like, the website's use of the word "reasonableness" ... It's accurate, but very awkward. If it helps, it might be easier to think of "さえ/さえない" as "even including/not even including" and "でも/でもない" "of course even/of course not even". ... The main point is that the "even" of さえ is surprise or disbelief, while the "even" of でも is acceptance and natural. "I can't even see my hand in front of my face!" would be translated with さえ, while "Even my baby brother knows the answer!" would be translated with でも. The reason the website says 私の日本人の彼氏は「I love you」とでも言ってくれない。is not correct is because the speaker should not be trying to imply that "not saying I love you" is reasonable/acceptable. I am not actually sure that is how it would come across, to a native Japanese speaker. It seems, as you say, not a bad sentence (to me, also still coming to grips with Japanese) but it does seem weak. ... and it might carry a sense of "among the (many) things he won't say to me is "I love you"." さえ/さえない would definitely give a better sense of the speaker's frustration. I hope my rambling explanation helps.