Some questions about Spanish

1. It's briefly mentioned on the page that "these bonuses [i.e., the 6-7-8 and 7-7-7 not counting Super Sevens] usually do not pay after doubling or splitting, but some casinos allow it." I know that Foxwoods and Twin River are among these (splitting, that is, not doubling - and the 5+ card bonuses as well), and looking at the rules posted on their page, the Casino de Montréal is as well. I was wondering, does this make a significant difference to the edge?
2. I can't help but notice that the S17 table says to hit 6-8 offsuit with a dealer 5 and 7-8 offsuit with a dealer 3, but not a four-card 14 or 15 with the same. Why? What advantage is there? All I can think is that it's somehow composition-dependent, in which case I can't help but think there's a better rule to follow.

If you look at the payout section, you get a bonus on 6-7-8 even for offsuit.

Hence the potential bonus can shift the favourable option to a hit.
If you have 6-8, you hit the bonus when drawing a 7. You don't do that for any other 14. The additional EV from that may exceed the EV for standing for some hands (like against dealers 5 in your example). Same with 7-8.

You get the same bonus for a five-card 21. That's why so many of the spots on the grid say "hit with x or more cards."

Yes you are right, didn't see the 5-card 21. In an infinite deck you should not expect a different EV for hitting 6-8 and hitting a 4-card 14. So either it's composition dependent, or the strategy guys are using different rulesets.

1. I think many casinos do pay the bonus after splitting, but not doubling. This includes Casino Montreal.

2. I believe this is composition-based. A 6-8 implies a running count that is slightly positive, while a 4 card 14 implies a running count that is more positive. The play is borderline (with a index right at balanced for both plays in Walker's book). I assume this means that you can hit the 6-8 for the 3-2 bonus but not the 4 card 3-2 bonus because of the count. Same applies to 7-8 vs 3.