[cliverlong]

1) If the cell nucleus is opaque how do we know the chromosomes are single strand during interphase?

I don't think the cell nucleus is completely opaque.  During interphase, the nuclear envelope is present which makes the nucleus appear more dense than other cellular structures.  Chromosomes, however, are fairly dense structures and they can be easily stained by various fluorescent dyes so you would probably still be able to see chromosomes if they existed during interphase.  The fact that DNA staining dyes are spread diffusely throughout the interphase nucleus argues that most of the DNA is not significantly compacted.  Furthermore, electron microscopy on interphase cells has probably revealed similar conclusions.  Despite not being compacted into chromosomes, some regions of DNA still exhibits some level of compaction into heterochromatin.  The determining exact structure and amount of compacted DNA is still an area of active research.

2) How does the pro-phase chromosome duplication happen?

Chromosome duplication occurs during interphase, not prophase.  In order for DNA polymerase and the other proteins involved in DNA replication to access DNA, DNA need to be uncompacted.