heaven /heaven

1a. Up to now, Germany seems as a safe heaven for capitals.
1b. Up to now, Germany seems as a safe haven for capitals.
2a. This city was regarded as a safe heaven for the refugees.
2b. This city was regarded as a safe haven for the refugees.
Please correct all.

I’m not sure what you mean by ‘capitals’ unless you mean ‘capitalists’.

‘Heaven’ is incorrectly used.

It is possible to say:
Germany seems to be heaven for capitalists.
Germany is regarded as heaven for capitalists.
but these do not carry the same meaning as your sentences using ‘haven’. They just mean that the country is an ideal place for Capitalists.

In 1b ‘as a’ is also incorrect:
Germany seems (to be) a safe haven for capitalists.
However, as you have prefixed this sentence with ‘up to now’, your tense is also wrong and needs to change:
Up to now, Germany has seemed to be a safe haven for capitalists. )For preference, I would use ‘appeared’ rather than ‘seemed’.)

To me, ‘a/the heaven’ would be the correct usage as the reference is metaphoric.

Can anyone use ‘safe heaven’ and ‘safe haven’ interchangeably?
Please comment.

The indefinite article is okay, but not necessary. I cannot imagine a scenario where the definite article would be preferable.

Well, I can. The indefinite article is preferable when what is likened to is accepted as or considered to be specific/definite/unique in the world or in the region as in:
(1) XXX is the heaven on the earth for tourists of all times and climes.
(2) YYY is the Shakespeare in Sanskrit.

The first example would not take the indefinite article:
XXX is heaven on earth for tourists, at all times and in all climes.

For the second '… the Shakespeare of… would be used with a place:
YYY is the Shakespeare of Mumbai. (The equivalent of Shakespeare but in Mumbai rather than Stratford-Upon-Avon.)
YYY is Shakespeare in Sanskrit.

So you say.

You can believe what you wish. I know what a native English speaker would make of it.

Yes. And don’t expect others also to be like native speakers. That is unreasonable. BYE!