4* Units to merge

Usually I inherit some skill for make space, but this time i noticed that there is some units that can merge until +10

Case #1 -- Eliwood 4*+10
5* +HP-Res x1
4* Not good IVs x5
3* +Atk -HP x1
3* Not Good IVs x4

With the access to Blazing Durandal, worth it to inherit Durandal to the +Atk -Hp unit and merge the rest into 4*+10 ??

actually my Red spots in Horse Emblem are Xander and Eirika, but at least he can work on CC or TT

Case #2 -- Gwendolyn 4*+7 (and waiting for more)
4* +Res -Spd x1
4* +Res -Atk x2
3* +Def -Res x1
3* +Atk -Def x1
3* Not good IVs x3

My Armor emblem still on process, with blue spot for 5* Effie. with out fodder for close counter, vengeful/bold fighter. and reviewing builds for Gwen also without Berkuts Lances. like provisional unit for Armor emblem and back up for CC/TT maps, worth it to merge? or upgrade one to 5* to upgraded killer lance+ and pass to the merge 4*

Case #3 Sophia 4*+5
4* +Atk -Spd x1
4* Not good IVs x2
3* Not Good IVs x2

For fill just some maps, last priority.
Probably I use some Fortify Res 3, before I think on merge.

Overall any idea/suggestion or recommendation