Since the Juarez treasure chamber is filled with sand in Bound in Blood ending, why is it open and free of sand again by the time Billy gets there in the original game?
There may be several treasure chambers with different traps in each one. Or Spooky Aztec Technology converted the sand into those infinitely spawning boulders.
What we originally thought to be the "Treasure Chamber" in the original game is actually revealed to be an antechamber (a "lobby" of sorts) for the main treasure chamber in Bound in Blood. If you watch the cutscene before Chapter XV in Bound in Blood, you will see that Ray and William actually walk right through the original game's "treasure room" on their way to confront Thomas. So really, it's justified that we never see the "real" chamber (from Bound in Blood) in the original game.
Why don't Ray and Mendoza recognize each other in the original game (when the latter poses as a Texas Ranger), even though Mendoza hardly changed and Ray still wears his trademark cuirass (which Mendoza notices)?
I think you have Juan Mendoza (Juarez) confused with his henchman, Ty Stewart. Stewart is the one who poses as a ranger, and he also happens to notice Ray's cuirass and points it out, whereas Juarez (Mendoza) acts as though he has never seen Ray before. Likewise, even though Ray never actually says "Who are you?" to Juarez, it's also implied he doesn't know Juarez either.
Further confusing matters, references WITHIN THE ORIGINAL GAME seem to imply that they do in fact have a history (an example would be Ray's remark: "So, events have come full circle."). Also, Ty Stewart is probably only in his 20s or so (too young to have seen Ray in action). So, who told him about Ray? My guess would be Juarez (it makes the most sense) but then there's that annoying problem of Ray and Juarez not knowing each other...
However, this plot hole really only exists in the first scene when Ray and Juarez meet, with no reference made to their history (or, rather, their apparent lack of history) ever again, so as long as you can get past that initial face-to-face encounter, it isn't a huge, gaping plot hole anymore.
I have my own thoughts on this, but it has bothered me for months now: If the ending of Bound in Blood is about Ray and Thomas realising the value of family, then why the Sam Hill are they such DICKS to Billy Candle in the first game?
Because Billy is not their family. He is the son of Juarez, the man who they (especially Thomas) hate vigorously. Also, they named him after William (probably before realizing that he is not Thomas' son), and they probably saw him as an impostor of sorts as he grew up. Not to mention that Thomas obviously had a drinking problem and that can cause problems even in normal families.
That's along the lines of what I was thinking. Marissa told Thomas in a cutscene in Bound in Blood that she was pregnant, so I'm certain Thomas already knew he wasn't the father. But the name/impostor idea is an interesting one. It's the Mood Whiplash that gets to me more than the implausibility, really.
Why does Ray (and in the sequel, both brothers) agree to so many quick draw duels ? The only point of a Western duel is to be able to claim self-defense (i.e. the guy drew before you but you shot him first), but Ray was busy murdering entire towns 10 seconds before so it's not like one count of self-defense out of 500 is going to matter much...
Maybe they just wanted to keep their skills sharp?
Murdering those other guys took time and effort and lots of ducking behind cover. In a quickdraw duel, your opponent is right in front of you. If you're reasonably certain you're the faster man and better shot, a duel is just more convenient.
I would really like to know where Ray got his cuirass. That's not something you'd commonly see on the streets in the late 19th century American South.
He didn't have it in Bound in Blood (did he?), so he must have gotten it during his time as a preacher... A donation from an elderly congregation member, perhaps?
He actually does have it in Bound in Blood, and it still has the cross on it, which means the cuirass isn't actually meant to be the "religious" armor it was originally implied to be.
How did Ray become a reverend between the end of the second game/start of the first one fast enough to marry Marissa and Thomas off before Marissa had her baby? (I like to think he threatened someone at gunpoint for the license)
It was a lot easier to become a priest back then than it is today. If ever there was an exam of some sort, he'd probably listened to enough of William's spiel to pass.
At the end of The Cartel, why does Kimberly slap that woman after she gives her a medal?
You mean Shane Dickson, Kim slapped her because she KNEW that Dickson was dirty and was theone who sent a drone to kill them earlier.