Minecraft Java Edition – How to Create a Fog Effect with Stained Glass in Minecraft 1.14.1

It is unadvised to have your login and password in the link. As far as I know and am concerned, The link is sent to the client and then downloaded from there. Through that, someone may look in the logs/console of the client and figure out the source of the download. Most may not notice but those that monitor traffic through a personal firewall may have that flagged out as suspicious and thus revealing that link.

It is advised to upload the texturepack elsewhere such is on dropbox or some other public hosting site where the user can download without compromising your server security.

As of 1.8, in un-modded and un-textured minecraft, there is no block that fits your requirements. Probably the closest you'd come is a Snow Block, a Wool Block, a Quartz Block, or, as you said, an Iron Block. The Quartz is extremely close to pure white, closer than the iron block. However, you should look into downloading a texture or a mod that may include a purely white block if none of these are close enough.