To answer your first question the actual part number for the manifold you need is 59023700. If you check out the parts diagram in the owner's manual you will see that there is only one type of manifold for this filter regardless of it's size (24, 36, 48, 60 sq ft, etc). Here's the link for the owner's manual see page 9, I will also include it on the bottom of the page:
http://www.pentairpool.com/pdfs/FNSPlusOM.pdf

The answer to your second question should be no. The grids you purchased are most likely universal grids. The same ones used in most types of Diatomaceous Earth filters. There should be no reason you can not use the grids you already purchased, unless the guy at the pool store sold you the completely wrong grids. That isn't likely however, as you were able to put it together.

As far as the sight glass goes, I understand where he is coming from saying the whole multiport valve needs replaced. You could attempt to remove the existing sight glass by gently tapping out the cracked pieces. Just a suggestion and not and industry approved method as you risk damaging the threads that hold the sight glass in. As far as an epoxy to use, I don't know of any off hand unfortunately.

I'll post a link below so you can see the manifold you need. It isn't an OEM Pentair manifold, but, I have used them on hundreds of filters and had zero issues. Plus the cost will probably be much more to your liking!

http://www.frooglepoolandspasupplies.com/custom-molded-products-de-filter-manifold-59023700-25357-600-000/

Hope this helps!

Here is a link that might be useful: Link for FNS Plus Owner's Manual