> Still, booting  with  b sd(0,0,7) gets me 
> 
> sun3_save_symtab: bad magic  

All this means is that the kernel cannot find DDB's symbols.
This is normal if you boot using the SunOS /boot and you do not use
dbsym when building the kernel.

> root on sd0h
> swap on sd0b
> dump on sd0b
> 
> I get
> 
> panic: cannot mount root
> Stopped at            0xe07d6fa:  unlk  a6

It is quite possible that the kernel must have root on the a partition,
but this is unlikely, esp. since the print-out above is correct.  Also,
booting with root on the b partition is required for miniroot support.

Are you sure that the filesystem on sd0h is intact?