lol sorry but I really don't think you need to use the spoiler thing, this is a spoiler convo anyway!! But each ot their own.
Nah it's cool. The manga and anime both have their strong points; but you're right they do emphasise the Isabel relationship way more and don't give Farlan a lot of screen time. However I think again it's a case of they had to choose one in order to at least properly develop it. I also believe they went for Isabel because she has several parallels with Eren in terms of characterisation.
Personally I like the deaths in both; the anime for impact and the manga for heart wrenching oh god the feels. Like, in the manga Levi SEES them die properly, like, he was there and within reach type of thing, so in many ways I found that MORE heart-wrenching because in the anime its more 'he wasn't there and so couldn't do anything' type of deal. But as I said, both have their merits.
I think you have a good point on the reaction thing, but again I feel its all about time. Like in the OVA they didn't have time to do the calm demeanour and have him kill several titans etc. etc. but in the manga it was able to develop that. I like his reaction in both, and I feel like the OVA captured what would have really been going on inside his head. I also feel they were going for that moment as 'The birth' or the Captain Levi we meet in AOT as from that point on he has nothing worse to lose type of deal.
The BIG thing for me was the way the OVA butchered Erwin's character at the end, making him mallicious and cruel at the end instead of open hearted and desperate for Levi to understand. Like... in the manga I really felt him rooting for Levi, like he wanted the man to understand who the real enemy was, and that they could keep fighting back type of deal. Whereas the anime is more 'haha ur friends are dead, how pathetic, kudos to you for living, now don't be a pussy and come with me forever'. So I was really annoyed when I read that ending in the manga. It was that difference that made me really want to read the AOT manga to find those changes too.