Proper use of Surrender indices

Hi all, I'm looking for advice on the proper way to apply the late surrender indices. For context, I play 2D, Da2, DAS, H17, late surrender offered. Under these rules, basic strategy says to surrender 16 against the dealer's ten or Ace. However, according to I18, I should hit 16 when the count is negative, and stay when it is 0 or positive. Which rule wins out? I'm going to guess in the vast majority of situations, it is best to surrender. However, is there a low enough count where it makes sense to hit 16? Or a count where standing on 16 against ten and Ace ever makes sense?

Similarly, 15 vs the dealer's 10. Basic strategy says to surrender. However, I18 says hit when the count is less than +4, and stand at counts greater than 4. And finally, F4 says hit if the count is less than 0, but surrender if the count equals or is greater than zero. Again, I'm assuming if surrender is offered, it takes precedence as the correct move. Therefore the F4 recommendation trumps both the basic strategy and I18 recommendations. Is this correct?

Yes. When surrender is offered it takes precedent. You should still know the indices though bc if it's a 3 or 4 card 15 and 16 and you don't have the option to surrender then you'll need to use the indexes.