So as it turns out, the compile error was an easy fix... just a default that neded to be over-ridden in the manifest (only MS and approved vendors get FullTrust...) oh, well.
Releases · Superfly-Inc/ShowKeyPlus · GitHub
Passed Windows App Certification with flying colours 
Code:
Windows App Certification Kit - Test Results
App name:
ShowKeyPlus
App publisher:
Superfly Inc.
App version:
1.1.7.0
App Architecture:
neutral
Kit Version:
10.0.16299.15
OS Version:
Microsoft Windows 10 Pro (10.0.16299.0)
OS Architecture:
x64
Report time:
03/01/2018 19:22:44
Overall result: PASSED
User account control (UAC) test
PASSED
User account control run level
Package compliance test
PASSED
Application count
PASSED
App manifest
PASSED
Registry checks
Windows security features test
PASSED
Banned file analyzer
PASSED
Private code signing
Supported API test
PASSED
Supported APIs
App manifest resources tests
PASSED
App resources
PASSED
Branding
App Capabilities test
PASSED
Special use capabilities
Windows Runtime metadata validation
PASSED
ExclusiveTo attribute
PASSED
Type location
PASSED
Type name case-sensitivity
PASSED
Type name correctness
PASSED
General metadata correctness
PASSED
Properties
Package sanity test
PASSED
Platform appropriate files
Results for optional tests
The tests in this section are informational only and will not be used to evaluate your
app during Windows Store onboarding. Investigating failures is recommended to
ensure that users will not be impacted.
Digitally signed file test
PASSED
Install signed driver and executable files
Package compliance test
PASSED
File association verbs
Debug configuration test
PASSED
Debug configuration
Package sanity test
PASSED
Archive files usage
PASSED
Blocked executables
Code:
Please note: Full instructions for installing the certificate under issues https://github.com/Superfly-Inc/ShowKeyPlus/issues/14
Additional info: https://www.tenforums.com/software-apps/2577-showkeyplus-post1204091.html#post1204091
Once it's installed further Appx's will install as an upgrade and separate certificate installation will no longer be necessary.
Update:
V1.1.1.0
Correction: Loading hives is NOT fixed yet - even with the change from HKEY_LOCAL_MACHINE to HKEY_USERS (loading hives can only be done with elevation which is not allowed - need to implement an alternate method)
- Removed elevation
V1.1.2.0
Fixed dll compile error.
Signed Appx
V1.1.7.0
Fixed offline registry access (using offreg.dll)
- It will redirect to Regback folder with auto-discovery (forward arrow) - still need to update the title in the ''Retrieve Key'' dialog to that effect
The config/SOFTWARE hive is owned by BUILTIN/Administrators which cannot be accessed by standard user - whereas the Regback/SOFTWARE hive is owned by System which can.