You can create a search on item with column with joinId as file and fetch name, url or inernalid like below,

search.lookupFields({
  type: ITEM_TYPE,
  id: NS_INTERNAL_ID,
  column: ['file.url', 'file.name', 'file.internalid']
});

You can use workflow to set date when status of opportunity is changed(Work in Progress). This will be quite easy than writing a script to do the same.

I haven’t implemented this in NetSuite but it should be something along the lines as described/answered in this stackexchange post.

log.debug or any other type of log methods does not work in Client script. You need to use console.log in client script.

You first need a relation between PO and SO(as @suitemike suggested) but since in your case PO are updated and that info needs to be back flown on SO, you only need to set/select reference of SO on PO and whenever an update is made on PO, you can make it flow to SO using workflows (in real-time/scheduled) or use some periodic (scheduled/map-reduce etc) scripts.