Welcome to the world of Hammer, a cold world with bugs and frustrating moments, but once conquered, the end result will put a nice grin on your face when you see your finished level come to life
1- As FMPONE said, use bspsrc to decompile bsp maps into vmf files. I bet you are aware of this issue but always keep decompiling others' maps for educational purposes and personal use. If you intend to modify or edit the map, you need to ask the original author's permission first
2- It is very common in Source to group several detail brushes into one func-detail entity like Spence mentioned. If you come across this scenario and want to select individual brushes then you can always select "ignore groups" (top toolbar) or "select groups/objects" (right tolbar) to temporarily disable grouping thus allowing you to select individual brushes regardless of their initial group and without altering the state of the group.
3- As Spence said, you can open the texture browser and select "only used" to filter textures that were only used in the map currently loaded. You can also use keywords to filter textures: typing "office" or door" in the texture browser will filter textures from cs_office (or city 17 office) and doors respectively.
Hope this helps and good luck in your exciting endeavor.