OK, I've added another omnibus sense to 影.

@@ -13,4 +12,0 @@
-</k_ele>
-<k_ele>
-<keb>影</keb>
-<ke_inf>&sK;</ke_inf>

陰に隠れ	        93,907		
影に隠れ	        66,507		
ドアの陰	        1,795		
ドアの影	        2,126	
陰のある表情	525		
影のある表情	555

I don't think it's necessary to split sense 3. It's not unusual to have glosses with different connotations for the same sense.
影 is commonly used for senses 2-4 (but probably less so in edited prose). I suggest either adding 影 as an iK form or adding those senses to the 影 entry with a "non-standard; usu. written as 陰" note.

@@ -35 +35 @@
-<gloss>background</gloss>
+<gloss>the background</gloss>

WRT adding 影 as sK, I think that obscures the fact that the alignment with 陰 is really just sense 1. Maybe an xref to/from sense 1 of 1590145 would be better.
For sense 3, I think the "behind someone's back" can stay. This dictionary is really for people coming *from* Japanese and the nuance should be clear.

Just encountered the "behind the scenes"/"in the background" in 陰ながら応援, and most terms using this seem to carry a fairly positive nuance.

Any chance of splitting out, and/or relocating "behind someone's back" from sense[3]?  I understand that these are from one single sense in Japanese ("from a place not physically visible to the target").  But certainly in English these are different senses(behind someone's back being idiomatic with a harshly negative nuance). It feels very odd to see them side-by-side, and I suspect it would be misleading for a non-native-English user to see them this way.  "behind someone's back" also suggest that the target is not aware of what is happening, but this isn't necessarily the case (and often isn't the case) with "background" or "behind the scenes" uses.

Equal parts support and slander:
陰で悪口	2814	26.9%
影で悪口	2540	24.3%
陰で応援	2073	19.8%
影で応援	3021	28.9%

I didn't make any actual changes myself, because it seems a fair number of sentences would have to be corrected to match up with such a change...

I stuck 影 as [sK] here, as plenty of expressions using these senses will be written with this kanji, whatever is prescriptively correct. It's really quite a jumble. (I've seen Japanese articles saying "some people write with 影, but that's wrong, they should use 陰".  This only appears, of course, because 影 is used all the time... see, for example:
陰ながら応援	98285	68.5%
影ながら応援	45211	31.5%　<- that's a bit much to dismiss. 

Not sure what we should do about it, but it would be odd of use to not record such a prevalent usage.

@@ -12,0 +13,4 @@
+</k_ele>
+<k_ele>
+<keb>影</keb>
+<ke_inf>&sK;</ke_inf>

Many of the sentences indexed to sense 2 were actually sense 3. I've reindexed them.