@Bradley
Are you aware that S3D 3.34 has an update to Shell?
Firstly for the information to the Community, this is about this specific Design only.
Tap on the the Face you wish to remove > Tap on the Dimension Selection next to the Double Arrows > Type in 1 [mm] and you have the result you are looking for:
Dragging the Double Arrow and snatching a ScreenShot you could capture this image, but it is only a captured Image it reverts to 0.0mm:
However that is as far as it goes, Direct Dimension Input only works up to 1mm.
It would be good to understand the problem with this Design.
Certainly things are awry, compared to the Outer Surface the Inner has additional Lines and ‘artefacts’:
OUTER INNER
Attempts to use a Copy side by side with the Original, for comparison purposes, and then Shell one Body failed. Hitting the Green Check Box immediately changed back as if it had never received an input, i.e., nil action from the S3D Icon to indicate anything was happening.
The problem seems be connected to Splined Closed Sketches, given that all Closed Sketches created without using Splines have been successful?
Please tell us if you are able to get the result you wanted.