1- Yeah, Old Yuna started to be more, what's the word, polished(?) after her backstory (so Ep.4?). Before that we had little reason to like her since she was being bitchy. She did look like your average Tsundere(?) that will shower you with insults with double meaning.
Like, I understood that her being all bitchy, more down to earth and sometimes brutal with the MC was to balance the rest of the girls that showered the MC with love and keep him grounded and remind him that he's weak and won't be able to protect his new peaceful life like he is (show him the ugly side) but the meaning doesn't matter if you don't like the person conveying the message.
Only after her backstory does she start to make more sense when it's shown that she knows who the MC would have to fight and what they're capable off, but by that time many gave Yuna a big "no, thanks" and I can't blame them.
2- I liked her because she was mysterious but as you said: If she knew about the people both her and the MC saw as enemies, why did she have to beat around the bush and only check the MC's power in EP.5?! She should've challenged him after they met and MC was bothering her with questions.
3- MC meets Yuna on the porch: Hey, I want to know why you act like this.
Yuna: You're annoying. If you want to know about me, then show me you're strong enough to know.
MC loses -> Yuna steals a kiss -> Tells MC to grow stronger before asking her again -> MC is motivated to train and make her his
You know, like it happens in ep5
4- This is why, even if I miss the old story, I believe the rework will go smoothly. While in the old one the characters were new and didn't really have an end goal, with the release of MBIML it gave everyone a set goal. Every character is more full of life after the rework, which made me await every new update for both LCC and MBIML sine I know they'll be good.
5- Damn, it's 3AM... gonna look for grammar mistakes later