Synonyms of いかに

Question

Could the word いかに always be replaced by どんなに／いくら／どれだけ／どれほど／なんと in structures like

1. いかに～か
2. いかに～ても／でも
3. いかに～とも
4. いかに～ことか

Basically, what I would like to know is if いかに and どんなに／いくら／どれだけ／どれほど／なんと are considered synonyms or there is any difference.

Thank you so much in advance for your help!

Answer 1

1. • いかに [adj.] か "How [adj.] (it) is!" (exclamatory)
     = どんなに／どれだけ／どれほど／なんと
   • いかに [V] か "how to V"
     = どう／どのように／どうやって
2. • いかに [adj.] ても／でも "no matter how [adj.]"
     = どんなに／いくら／どれだけ／どれほど／どれくらい
   • いかに [V] ても "no matter how [V]"
     = どんなに／いくら／どれだけ／どれほど／どれくらい (how much)
     = どう／どのように／どうやって (in what manner)
3. = #2 except verbs' conjugation (also often become volitional form "no matter how (it) would/should [V]")
4. • いかに [adj./V] ことか "How [adj./V-ing] (it) is!" (explicitly exclamatory)
     = どんなに／どれだけ／どれほど／なんと
     (only such verbs that is semantically stative could be used)

Basically いか is an older word that has double face of "how much" and "what manner/type". To make things more complex, it is often involved in idiomatic or largely fixed expressions, which you have asked. There are only limited forms how いか is employed today:

• いかに: (degree) どんなに／いくら etc.; (manner) どう／どのように etc.
• いかなる: どんな／どのような "what kind of"
• いかが: どう "how about; what about; what like"
• いかん (almost a particle; better see the link)
• いかんせん "unfortunately"