Bike maintenance - small bits

 Mountainbike:

Bell slipped, fixed back up
Keeping light attachment to side for safety.
Water bottle carrier weak plastic broke more, have it fixed with rubber from old tyre
Front mudguard from Maeve's bike take off - plastic bottle mudguard made smaller.
Right gear shift (rear derailleur) as always can be difficult to shift, something doesn't catch inside - need to fix SOME day
Preparing to put on front derailleur.

Hybrid:

1. rear derailleur bottom stop adjust to stop chain hopping down and stuck
2. front mudguard is rubbing, it split a while ago! and is held together with duct tape. Loosen out and give more space at rear of wheel, raise a bit under handlebars.
3. front brake pad check, pretty worn and spring missing bits, looks like it is rubbing one side. Replaced with some cleaned up old ones - won't last long!

QUESTIONS

Do I need a shimano quick-link connector? Or can I just pop out rivet and put it back in? Can I re-use the rivet pin?

https://www.bike24.com/p2187164.html

https://cyclingtips.com/2019/01/chain-quick-links-guide-to-easy-connection/

https://www.bikeforums.net/road-cycling/1009718-reusing-chain-pin-will-i-die.html

https://www.roadbikereview.com/threads/reusing-shimano-chain-pin.89914/

https://bicycles.stackexchange.com/questions/43236/putting-pins-back-into-chain

https://bicycles.stackexchange.com/questions/24339/how-do-i-re-insert-a-chain-pin-that-has-been-pushed-completely-out

https://bicycles.stackexchange.com/questions/28773/do-i-really-need-a-magic-link-or-can-i-just-replace-the-pin-on-my-chain

https://www.bikeradar.com/advice/workshop/how-to-replace-a-bicycle-chain/

https://www.youtube.com/watch?v=OdCNs7nFw60&t=43s

https://singletrackworld.com/forum/topic/11-speed-shimano-quick-link-compatibility/

How to put on the derailleur ...

https://www.sheldonbrown.com/derailer-adjustment.html#chain

https://www.bikeride.com/shimano-hg-ig-chain/  How to install

https://www.youtube.com/watch?v=42_tKpE6PhI  how to change front derailleur