Don't worry about picking my brain at all, I'm always happy to help you succeed with the asset! You are doing great.
1. The Easy Way to Lock the Slide Back
You don't need complex blueprint math to lock a pistol slide back when empty.
Right-click your pistol's empty/reload animation and click Create Anim Montage.
Open that Montage, find the exact frame where the slide is pushed all the way back, and pause it.
Right-click the timeline at that frame and click New Anim Section (call it 'HoldOpen').
Set the montage loop settings so it stays stuck on that 'HoldOpen' section. Now, just tell your blueprint to play this section whenever your ammo variable hits 0!
2. The Golden Rule for Quick UE5 Retargeting
When you open the IK Retargeter asset, make sure your two character models look like they are standing in the exact same pose (usually an A-Pose or a T-Pose) before you click export.
If one character has wider shoulders or lower arms, use the Edit Retarget Pose button to manually move their arms until they match perfectly. This stops the weapons from looking twisted when you swap them!
Maybe you can provide me the screenshot of your issues so I can help you solve it