I can't answer much of your question, but they may simply be synonyms. I think "ikiwa" is possibly a bit more clear, since "kama" can also mean "like/similar to". Also, "ikiwa" is itself the conditional form of the verb "kuwa", with "i-" (class 9, singular N class) as the subject. It basically means "if it is" on its own.

I think I saw somewhere that there's some variation in the verb tense used in the "if" part of the sentence. I think they should explain it a bit better in the tips and notes section.

I am a non-native speaker, so FWIW, I feel more comfortable when a simple tense (future, past or present) follows "ikiwa": ikiwa atakuja, ikiwa anakuja, ikiwa amekuja, ikiwa alikuja. I'm not very comfortable with "ikiwa akija" -- that I would just shorten to "akija". Could be a regional thing, or a "native speakers aren't bothered about it" thing. I am fairly certain I once heard a native speaker say "Iwapo kama watapata bunduki .." (If they get guns ...)

I think that you don't have to say "Ikiwa" if verb with "ki" is following. In the presvious lesson of conditional tense there are sentences without "ikiwa" or "kama" like "Akipika nitakula."