I canīt answer your 1.), because a) I donīt remember the scene in detail, and b) it might be a case of bad writing. Happens.

2.) Again, if I fought a turtle-monster, Iīd make the same assumptions that Ernestie did. Would I be guaranteed success? No. But itīs a reasonable assumption. Youīre talking about it as if it was an outlandish assumption that shouldnīt have been realistically possible. Disagreed.

3.) This is episode 2. Tanya was OP for most of the anime. How about we wait and see what happens?

4.) Youīre answering your own question: Itīs a fantasy world. For Ernestie, this still isnīt real. Admittedly, heīs been living in this world for a couple years by now, but we donīt know how much heīs accepted that as "real". Evidentally, he spent most of his time learning high-level robot-stuff so he could pilot a mecha as soon as possible. Thatīs his one and only goal. Some fantasy knights dying? Who cares, theyīre not real, not in the sense of "my" real world.

I will say this: If this anime wants to aspire to greater heights, it WILL need a moment of humiliation for Ernestie. Him losing. A dear friend dying. Stuff like that. If this is 12 episodes of Ernestie pwning everyone and everything, itīd be bad in hindsight, yes.