Both of your sentences are valid, but the meaning are not exact what you think them to be.
わからないでも言って下さい
Please say it without understanding.❌
Please say it even if you don't understand.⭕️
The second sentence is good, except that you might have mixed up 2 and 3.
でも should be considered as で + the particle も meaning "even/also". In the above sentences, わからないで is a whole word, aka the te-form of わからない. So it's わからないで + も. The second sentence, the te-form of 二時間おき is 二時間おきで(the te-form of nouns and な-adjectives), and the sentence is 二時間おきで + も.
Since も means "also/even", わからないで + も is literally "even if not understand". 二時間おきでもいいです is literally "even if every two hours, is good(いいです)".
Sometimes you see でも at beginning of sentences meaning "but", but that's just short for それでも, literally meaning "even if that", where it's それで + も, それで being the te-form of それ.
でも、俺は行かない.
But I will not go.
(Literal) Even if that, I will not go.
Similarly, sometimes you see sentences begin with では, or だから, or だけど, etc, they are all contractions of それでは, それだから and それだけど.
I'm not going to explain what is te-form or how it's used here in detail, because that will make this answer way too long, and there are already lots of resources dedicated to it online. But in short, you need the te-form when connecting a clause to は or も
全部食べるもいい ❌ 全部食べてもいい ⭕️
行かないはダメ ❌ 行かなくてはダメ ⭕️