This is a nice interior, and generally everything is placed very well; you have a good grasp on which meshes can’t simply be ‘F’ed into place, you’ve used gridsnap appropriately and other than one instance there is no style-mixing. The file is clean, IllegalToSleepHere is set, you’ve added a Northmarker and the lighting values are good. There are a few general problems that need to be addressed, and some floaters/bleeders, but this is a high quality showcase overall.

Firstly, some of the interior pieces you've chosen don't match together correctly; mainly in_hlaalu_hall and in_hlaalu_room pieces. Pay careful attention to the curvature of the roof. Most of the mismatched pieces can be fixed with equivalent versions from the matching set (in_hlaalu_hall_corner_01 instead of in_hlaalu_room_corner, for example), but a few places might necessitate a bit of redesign.

Tapestries should be positioned either with the rope knot completely out of the wall with only the wooden peg bleeding, or with knot and rope leading up to it completely bled out of sight. Move the tapestries until the rope knot at the top doesn't bleed into the wall/ceiling.

Remove the light by the window (or make a script for it), if lights representing exterior light are used they should be scripted to fade away at night.

Some objects have ownership set; these should be reverted to un-owned as ownership will be handled when the interior is populated.

sc_icarianflight is a unique item, replace it with a more generic scroll.

bk_ArkayTheEnemy should be replaced; this should only really appear in shady wizard houses/necromancer dens and the like. bk_BlackGlove should likewise be replaced as its use is generally restricted to Morag Tong affiliated interiors.

The TR_furn_com_p_bkend shouldn't be used in de_* interiors. Replace with de_p bookends.

The Ex_De_Shack_Plank_03 reference floating in the basement obstructs player movement a bit too much, try and reposition it to give the player an easier path.

This isn't important for your showcase, but generally key ids should be named to indicate the interior they belong to.

Use Wrye Mash to remove the dependency on TR_Mainland.esm

Here’s a complete list of bleeders/floaters that need to be addressed. Don’t be discouraged by the length; TR adheres to a very strict standard for interiors, and given the precise placement present in much of your showcase, I’m sure you won’t have much trouble addressing these:
 

apparatus_j_mortar_01- should be bled further into Furn_De_Bar_01
barrel_01_saltrice- bleeding slightly into floor
bk_Ashland_Hymns- bleeding into bk_ArkayTheEnemy
bk_BriefHistoryEmpire3- bleeding into bk_BriefHistoryEmpire4
bk_BriefHistoryEmpire4- floating
bk_BrothersOfDarkness- bleeding, should be rotated to lie flat against the bookshelf
bk_OnMorrowind- bleeding into bk_onoblivion
bk_onoblivion- bleeding into bookshelf
bk_specialfloraoftamriel- bleeding into bookskill_restoration, bookshelf
bk_words_of_the_wind- bleeding into bk_Ashland_Hymns
BookSkill_Alchemy3- bleeding into bookshelf
bookskill_restoration1- bleeding into bookshelf
common_pants_04- slight caspering visible at the back, bleed it into the table a tiny bit more
crate_01_empty_Float- corner is bleeding slightly into wall
ex_ai_boulder07- bleeding through wall, caspering visible
ex_ai_boulder08- bleeding through wall into neighbouring room slightly
furn_de_p_stool_02- floating, rotate until the legs are lying against the floor
furn_de_p_table_01- bleeding into floor
furn_de_screen_guar_01- these shouldn't bleed into the floor
furn_de_screen_guar_01- as above
furn_planter_01- floating, should be rotated to lie against de_drawers_02
Gold_005- bleeding into furn_de_p_table_02
Gold_010- bleeding into furn_de_p_table_02
in_hlaalu_door & in_hlaalu_doorjamb- need to be gridsnapped into place
in_hlaalu_hall_rail- some caspering visible, try scaling it up by .03 and repositioning until both ends are completely covered
ingred_comberry_01 - bleeding into misc_com_redware_plate
ingred_crab_meat_01- bleeding through misc_com_redware_plate
ingred_hackle-lo_lead_01- floating a little, rotate to lie against Furn_De_Bar_01
ingred_heather_01- bleeding into misc_de_bowl_redware_01
ingred_ruby_01- just barely bleeding into crate_01_food_misc01
ingred_scrib_jelly_01- this should bleed into a surface just enough for no caspering to be visible
ingred_wickwheat_01- bleeding into Furn_De_Bar_01
light_de_lantern_06_128- bleeding into furn_de_p_table_01
light_de_lantern_06_128- slight bleed into furn_de_rope_04
light_de_lantern_06_128- bleeding into furn_de_rope_04
light_de_lantern_06_128- slight bleed into furn_de_rope_04
misc_com_basket_01- bleeding into furn_de_p_bookshelf_01
misc_com_bottle_13- floating
misc_com_broom_01- bleeding into de_p_closet_02_empty
misc_com_metal_plate_07- bleeding into de_drawers_02
misc_com_redware_flask- slight bleed into de_p_closet_02_empty
misc_com_redware_pitcher- bleeding into furn_de_p_table_01
misc_com_redware_plate- bleeding into furn_de_p_table_01
misc_com_wood_fork- floating
misc_com_wood_fork- floating
misc_com_wood_knife- bleeding slightly, rotate until entire handle lies flat against floor
misc_com_wood_knife- floating, rotate until handle is flat against furn_de_p_table_01
misc_com_wood_knife- rotate until handle is flat against TR_furn_cutboard
misc_com_wood_spoon_01- bleeding into furn_de_bar_02
misc_de_bowl_redware_03- bleeding into furn_de_p_shelf_02
misc_de_goblet_04- floating
misc_de_goblet_04- floating
misc_de_goblet_04- slight bleed into furn_de_p_table_02
misc_de_pot_redware_04- bleeding into furn_de_p_shelf_02
Misc_Inkwell- floating
Misc_Quill- bleeding into De_P_Table_01
misc_skull00- bleeding into furn_de_p_bookshelf_01
sc_paper_plain- floating
sc_paper_plain- floating
silver dagger- floating
TR_bk_Aldugga1_MK- floating
TR_bk_Aldugga3_MK- should be rotated to lie flatter against bookshelf
TR_bk_Aldugga3_MK- floating
TR_bk_Aldugga6_MK- floating
TR_furn_bottleT_b02- bleeding into floor
TR_furn_de_p_lecturn- floating
TR_furn_de_Pitcher_broke05- bleeding into floor
TR_furn_de_rug_n04- floating
TR_furn_de_tapestry_17- bleeding into wall
TR_furn_de_tapestry_17- bleeding into wall
TR_furn_de_tapestry_17- bleeding into wall
TR_furn_de_tapestry_17- bleeding into ceiling
TR_furn_de_urn_broke07- should be rotated to lie flat

Address these issues and I’ll review your showcase again.