Luis tells Leon his name by pronouncing it with a Spanish accent, "Loo-ees," but Leon always pronounces it "Lewis." This is probably a mistake by the voice actor, but Leon might simply be more comfortable pronouncing Luis' name with an American accent. Luis doesn't seem to mind.
If Saddler's plan involved sending Ashley back to the US infected so he could manipulate the president through her, why didn't he just let her and Leon go on the first helicopter Hunnigan sent rather than have it shot down? If it's because they already knew about the parasites, they were intent on going back anyway and the only surgical procedure to remove them is on the island, and besides which the only reason they knew about the parasites at all was because Saddler blabbed it in true Bond-villain style.
Saddler explains that bit; he wanted to make some money first before getting on with taking over the world by bargaining for Ashley's release. It's also possible he thought that just letting Leon go would be too suspicious.
Not to mention that he ordered Mendez to capture both Luis and Leon alive (the letter in Mendez's house states this). Great move, letting Leon get chummy with one of the two people who could tell him about the Plaga.Also, if Leon and Ashley had managed to escape the village instead of getting trapped in the castle, Luis wouldn't have had the chance to give Leon the Plaga-suppressant drug, and he would have fallen prey to it in few hours.
Which would be a good thing right? The plagas clearly have something of a hive mind going on. The President's Daughter and the Hero of the Hour under your control when you're trying to overthrow the US government (or infect them with plagas) is a brilliant strategic move. Maybe he wanted to make it look difficult but he really did botch his own plan to get Ashley in place all on his own.
Why does Ada go about her mission in a flimsy evening dress? It can't be to use her femnine wiles to persuade Luis to help her; he's already agreed to work with her, so the tactical gear she wears in "Assignment Ada" (in which she is still hot) would make more sense.
...when is military gear hot?
When it's form-fitted and being worn by Ada Wong. You played the game and saw it, right?
It's constantly emphasised that Saddler needs Ashley alive for his plan. So why are there a bunch of times when the bad guys try to kill her? (e.g. Salazar's spiked ceiling, the drilling machine, the soldiers in the bulldozer sequence)
Because the bad guys know that Leon was able to survive most inhumane situations, like the bit with Mendez, so they're more testing him to see if he could be a possible second-in-command in case Krauser fails.
They also had a contingency plan in which Saddler's militia would invade the United States and bring chaos and destruction.
The Merchant. He's very obviously not human. No-one ever comments on this, and it's never explained why there's this blatantly-non-human person wandering around selling you weapons.
If you look carefully at his eyes, they're glowing like the other Plagas-infected Ganado, and his skin's also very pale. It's possible he has the parasite, but his strong willpower fought it off so he could help you.