I'm a native Hebrewer living in Israel, never heard טעים in this sense. Admittedly, I might not be hanging out in the right society. I don't know of a word in Hebrew slang which I'd say is the translation of "hot". סקסי is indeed used; I'd imagine it's the translation of "sexy"... Is there a difference in English between "sexy" and "hot"?
I think there are people who are hot (= aesthetically beautiful but I personally don't find them sexy/sexually attractive. ). The words in American English can be used interchangeably (I am a native speaker). I personally don't use sexy to refer to other women, unless it's about their clothing (that dress is too sexy for work) or I know a friend is looking for a certain look and wants confirmation. I don't use hot to refer to a book. I will say a book was sexy or the story was sexy. Hot in reference to story/book is usually referring to a big deal or very popular.
I don't think "hebrewer" is English. Native Hebrew speaker/ Hebrew is your mother tongue.
I think that tasty and delicious are accepted throughout. Yummy is quite colloquial/maybe a little childish, I'm guessing it didn't occur to them to add it as a standard translation. However, I think they would probably add it if you reported the sentences where it was not accepted.
Because apparently this is meant about food (contrary to what many comments in this discussion imply). In English, can you say about food "beautiful"? If you can would it mean normal "pretty" or something extremely exquisite (very fancy cake or a a chef's dish)? If the latter, in Hebrew you'll want a stronger word than "יפה".
I think it's uncountable "nikud" [although I've also seen it as nikudot, I'm not sure if using the plural form is a diaspora thing only or not (Jews outside of Israel). I'm learning Hebrew too, so I can't say with any authority if any plural form is correct. The course tips and notes uses the term "nikud."
As for the iPhone issue, are you using the Apple IOS keyboard and did you switch it to Hebrew?
I don't have an iPhone, I do have access to one... I would recommend that you install a third party software to remedy this issue. (the last person who asked, I advised throwing it (iPhone) out the window to be less frustrated... but my second suggestion, if you don't like the first, is to install a third party keyboard app... Like SwiftKey, or Davka App...
I use Swype keyboard on Android, which is no longer updated (it got bought out) but still works well. There was an IOS version, so if SWIFTKEY isn't helpful for you (I really think you'll like it though) you can search for Swype on third party sites and download it.
Let me know how you get on.