I am working on mirroring five meshes--all of them are part of the SPatio or SThalmor set (the bits that make up the Thalmor Embassy, basically). I am having a few issues, though, and I am hoping someone who knows more about it can help me out...

1)
I know how to load a Mesh into Blender, but it seems there are a number of 'unrecognised' blocks... I'm not actually at my Computer right now, unfortunately, so I can't give the exact block names, but one of them is a BSLODTriShape, and the others are something to do with Compressed Collsion, or something... For now I've been just deleting them, but is this going to break something later (I'm particularly worried about the collision stuff, since I'm obviously going to need to be able to walk on these platforms...)

2)
BSLODTriShape actually contains part of the Mesh--in the one I have been practicing on, it's a railing. Deleting it obviously deletes that railing, which I do not want to do. I 'fixed' that by making a new NiTriShape, and trying to mimic the others. It seemed to work, but I have no idea if that's actually "safe" or if it will cause issues later.

3)
The piece I am practicing with has 6 TriShapes, but when I import the nif into Blender, it has merged everything into two models--the railing is one, and everything else (walls, floor, steps) has been merged into a second object. When I apply a texture to this in Blender, the two objects texture independently as 'one' each. (so I can have the railings with a texture, and then the floor, steps and ground having another texture, but it being the same one for all of them as a single sheet, rather than individual textures... I hope that makes sense? :/ )
When I export that back into NifSkope, I have only 2 TriShapes: one for the railing, and one for everything else.
I'm especially concerned about this, since the Thalmor Embassy building looks to have about 20+ TriShapes, all with individual textures, so...

4)
I can get the textures to come up in Blender, but for some reason they are not appearing at all in NifSkope... I have no idea where I am going wrong here. I've been looking at several texturing tutorials for Blender, and the one I finally got to work is the one which leads to this issue--the others I could not even get to work in Blender, let alone NifSkope...

5)
Some of my 'faces' are being flipped when I'm mirroring the pieces. I've used NifSkope to revert them, but until I get the textures working I can't tell if this is going to cause other problems... Is there a way to prevent this flipping when I am mirroring the nif in Blender?

Thanks for any help anyone can give me

It sounds like your first foray into Blender, so do yourself a favour and use the latest version (currently 2.74) along with the http://www.niftools.org/projects/plugins/blender_nif_plugin/docs/user/install.html.

This will solve most of your issues, especially #4 of retaining textures. In 2.49 this requires sacrificing chickens.

You will not be able to import collision either way (#1). You'll have to remove that branch in Nifskope first, then recreate new collision for your modified mesh when you're finished.

BSLODTriShapes (#1 & 2) are another branch type that Blender does not deal with. You can work around as you have by first converting it to a regular NiTriShape for Blender, then converting it back in Nifskope after (set up the BS Properties levels the same as the original) or leave it as a NiTriShape. All it's doing is allowing the railing to be seen in the distance.

When you mirror a duplicate mesh in Blender (#5), the normals sometimes do get flipped. Correct this by having the object selected and hit CTRL+N - recalculate normals to outside, and that will fix it.

As for #3, Blender in no way combines meshes unless you tell it to. You probably have the import setting "Combine NiNode + Shapes into Single Mesh" selected on import. Uncheck that. The only option you need on import is "Import Extra Nodes".

Here's a tip: Unless you're going to change every single aspect of a large complicated mesh, don't try to import the entire thing to work on. Strip down the mesh in Nifskope until only the part you want to work on is left, then import and work only on that part in Blender. After exporting, copy that NiTriShape back into the full mesh and delete the existing one.