3. It seems this SRFI is created because SRFI-136 can't be implemented
without keeping compatibility of other record types.

I don't think so (but I am not the creator of SRFI 137). SRFI 136 can be implemented as a conservative extension of SRFI-9 and R7RS-small as much as SRFI 99/131 can be implemented as an extension of SRFI-9 (with field names being symbols).

SRFI 99/131/136 could be implemented using SRFI 137 as a common base, but they could as well be implemented on top of each other.

Cheers,

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_/_/
Takashi Kato
E-mail: ktakashi19@gmail.com