the problem is the imported mod which i only got one the DLC_MOBS
since the definition of imported from wrye bash is "Import means that some specific aspect of a mod is extracted from the source mod(s) and/or file(s), and then is applied in the patch over any subsequent changes."
it then means the mod will not fully do what its supposed to do
No, it means that the only intended change from the mod gets applied. Oblivion mods contain records, where one or more aspects of a record is changed. Say that modA changes the stats of a weapon and modB the texture. In this case, the intention is to only import the stats from modA and only the texture from modB. If you used the entire modB, its vanilla stats would overwrite the stat changes in modA, making the two incompatible, but by only importing the intended changes and nothing more, the Bashed Patch solves the incompatibility for you.
Some mods needs to be imported and deactivated (if the only intended change is what gets imported), while some mods (my Soulgem Magic is an example) needs to be imported and still stay active since it has some aspect that needs to win over later-loading mods (the imported changes) and other aspects that needs to stay active, but that WB cannot import or merge. In Soulgem Magic's case, it adds a new quest and some scripts and some new soulgems and none of this cannot be imported or merged so it has to stay active, but it adds scripts on the vanilla soulgems, and this should be imported to ensure that other changes done to the soulgems done by other mods don't overwrite the script addition.
But WB reads all this from BOSS' tags, so you get it right if you do as WB suggests.
after rebuilding of your bash patch should you use it as it is? or do i need to do changes like tick DLC_MOBS so it would be fully active?
No, don't do anything. Wrye Bash reads the tags from BOSS and therefore knows exactly what to do with the mods when it comes to importing/merging/deactivating, so just do as WB wants and you get it right.
also after rebuilding.. i did not remove any mod.. so why goes the game tells me that some mod is missing and that items may no longer be available?
Edit: if one mod has now been merged/imported and decativated, the engine sees that the mod is no longer active, and has no idea about parts being imported into the bashed patch. This message can safely be ignored.