Why has Chris buffed up so much by RE5? Among other things, he didn't have the strength to lift Parker quickly enough when he fell off the catwalk. Naturally, he doesn't want that to happen again.
After Raymond gets shot and appears to die, Parker refuses to go back to the Casino or Emergency Communications Room, citing the need to deal with something at the Bridge. He can still up to the Solarium, though, which is actually much farther from the bridge than either of the two other locations. It seems to be a subtle show of grief. The only time earlier in the game that Parker ends up in the ECR is when Raymond is there and the Casino has Raymond's body lying on the floor.
It seems unusual that the Fenrir enemies are much weaker compared to other dogs infected by the T-Virus, but then it's because they're infected by the T-Abyss virus (which originates from deep sea fish) and they're encountered in a snowy mountain area, not suitable for the infected creatures.
A lot of people make fun of Rachael's character design for being one of the least functional outfits possible, particularly for this series, but in a way, it makes sense. If you watch the Tokyo Game Show trailer, she clearly isn't combat trained, her notebook mentions she's not on the Zenobia by choice, and she's quickly cornered and killed. Everything that you would expect to happen to a woman with that hair, in that outfit, in a dangerous situation, happens almost immediately.
And as for that hair of hers? From the note she leaves behind she mentions having lost an eye. Let's suppose Rachael did lose an eye at some point on the boat. She didn't have any first aid on her, so she had to make do with using her hair to cover up the wound, and she perhaps didn't have the time to adjust her hair properly to cover up just one of her missing eye when monsters started attacking her.
Why didn't the vaccine save other people from the T-Abyss virus? It was only a prototype developed desperately and haphazardly so it might not have been complete. The vaccine helps Jill and nobody else because she already has immunity to the original T-virus, so the vaccine works with her existing antibodies. Whether or not this makes sense in Real Life is academic as Resident Evil depends heavily on fictionalized biology
The Emergency Communications Room seems like a pretty important part of the ship. One that should be powered at all times in case something goes wrong, such as the main power for the ship going out... so why is it that the ECR is only hooked up to the main power, when pretty much every other part of the ship (including the casino) can run off the emergency power?