check these 2 links ‘closely’ - they’ll usually fix most issues with retargeting:
https://docs.unrealengine.com/latest/INT/Engine/Animation/RetargetingDifferentSkeletons/
because sometimes it helps to set the skeleton like here (after doing the above):
https://docs.unrealengine.com/latest/INT/Engine/Animation/AnimHowTo/Retargeting/
you can also ‘show’ the skeleton & names in the viewport under the options at top, which might help
only having 2 spines might not be good sadly or may be hard to match up
personally, I would try not retargeting at all unless you need the default animations or the animation pack from learning tab in launcher.
you may only need to import them correctly if you already have animations, it’s what you have to do with blender unless you just like ‘pain’.
to make it short n sweet, retargeting to the humanoid isn’t necessary to use animations that you made yourself.
*note: always give details like what 3d software you are using etc. - it will help you find someone with the correct answer for your situation.
hope this helps
*edit: forgot to say, you can also just retarget to your own skeleton by setting ‘it’ as the humanoid skeleton. it will save you a lot of room as well as being able to share animations etc.