1) DX10 basically provides improved graphics and performance. To take full advantage of it you really need to go to Avsim where there is a dedicated forum for DX10. Members there have made great strides in completing what Microsoft started with the DX10 preview.
I would not mess with it until you have a stable FSX, then if you want to try it apply the Avsim patches. I personally like DX10 though there are still some issues with it and not all addon scenery and planes work with it.
2) The Win 7 version of uiautomationcore.dll can cause FSX to crash after repeated menu clicks. If you experience this you can place the Vista version in the root FSX folder, leaving the Win 7 version where it is, to resolve this specific issue. Note that there are two different versions and you may have to try them both to get the right one. One works for some people while others need the other version. Again, do not mess with it until you experience that specific crash.
3) Most every object in the sim is "painted" with textures. When the sim draws objects on your screen rarely do the pixels match up one to one between the textures and your screen. Filtering determines which method the sim uses to blend the pixels from the texture sheets to draw the object on the screen. Leave it on trilinear.
4) You certainly can save different setups if you wish, FSX allows this to be done. I cannot speak for most users, I do not have a bunch of different ones. For example, the prop controls used for a constant speed prop do nothing in a fixed pitch prop plane, so there is no need to not have them.
There is an addon called FSUIPC that is as close to a must-have as there is. It is free and many programs need it to communicate with FSX. If you choose to pay for the registered version you get many other capabilities including the option to set up plane-specific hardware profiles that automatically make settings changes for the plane you are flying.
WWOD---What Would Opa Do?
Farewell, my freind (sp)