IpadOS connect lightning to hdmi and usb together
So, if I understand right you have basically two set of questions:

1. can an iPad Mini (5th generation) be connected to a 4k monitor? if so, how? and what can I expect in terms of image quality/resolution?    

Yes, it is possible to connect a lightning-iDevice to a HDMI-Display using a Lightning Digital AV Adapter. It will mirror your display. The iPad Mini 5Gen has "1536 x 2048 pixels, 4:3 ratio" and that is what you will see in your 4k-Monitor. (for reference check this video)

2. given that the connction iPadMini-Display is working, can I have more USB devices attached?  

Unfortunately, no. And here is a video that shows it ^^
I really doubt there is a way to map the Lightning-pins to HDMI-pins AND to the USB2.0-pins simultnaeously
... if you had in mind to use a mouse or a keyboard, may be you could achieve that over a Bluetooth connection.



Side questions:
Why is my lightning to type-C-USB connection not showing any picture?
This is because of the diversitiy in protocols/standards. Lightning on that iDevice, in terms of USB is essentially USB2.0 and this USB standard does not support video/image transmission. The standard that supports that is a USB-C alternate mode which, at least, requires USB-C on both sides. (you only have one, on your display...)