PC
Thanks to /u/depreciated_ on reddit for their solution:
Run the following script in Powershell (run as Administrator):
New-NetFirewallRule -DisplayName "Destiny2-Solo-1" -Direction Outbound -LocalPort 1935,3097,3478-3480 -Protocol TCP -Action Block
New-NetFirewallRule -DisplayName "Destiny2-Solo-2" -Direction Outbound -LocalPort 1935,3097,3478-3480 -Protocol UDP -Action Block
New-NetFirewallRule -DisplayName "Destiny2-Solo-3" -Direction Inbound -LocalPort 1935,3097,3478-3480 -Protocol TCP -Action Block
New-NetFirewallRule -DisplayName "Destiny2-Solo-4" -Direction Inbound -LocalPort 1935,3097,3478-3480 -Protocol UDP -Action Block
Write-Host "Destiny 2 Solo mode activated. Press ctl + c to deactivate solo mode."
And then run the following when you are done:
Read-Host
Remove-NetFirewallRule -DisplayName "Destiny2-Solo-1"
Remove-NetFirewallRule -DisplayName "Destiny2-Solo-2"
Remove-NetFirewallRule -DisplayName "Destiny2-Solo-3"
Remove-NetFirewallRule -DisplayName "Destiny2-Solo-4"
This will force the game to block specific network ports, preventing other guardians from joining up in your fireteam. Do note as mentioned in his post that the script needs to be run while your ship is fully in orbit, before starting "matchmaking". After a few minutes, the game will automatically start the strike regardless if others join your team (in this case, won't happen), and you'll be free to play any Strikes solo! This applies to both Nightfall strikes, Nightfall: The Ordeal, any strikes in the Vanguard playlists, or Strikes started manually from each planet's map.
PS4
You can set your time back, preferably 1 hour. This prevents others from matching with you, and you can solo strikes this way. Don't forget to switch your system clock back to normal if you want to play with others again.
You described the differences pretty well.
In Destiny 1, shaders were normal items - they were not consumable. There was only one shader slot, and it applied to all your armor (weapons were not affected (related, most exotic weapons had one or two ornaments that worked similar to shaders, but were consumable)). You could change the shader at will, and the shader would not be destroyed. Later on in Destiny 1's development they added a shader kiosk where you could get a new copy of any shader you have ever found which would allow you to effectively use the same shader across all your characters.
In Destiny 2, shaders are now applied per item and can be applied to weapons, sparrows, ships, and ghosts in addition to armor. Shaders are also consumable - once you've applied a shader, you can't use it again. If you replace it with a different shader, the first shader is gone. When a shader drops, you typically get a few copies of it (I've gotten 3 at once, not sure if that is guaranteed).
The main issue some members of the community have with the new system is that in addition to being consumable (which means you tend to think about applying a shader, since you can't use it again), is that you can buy them with real money via the Eververse store. The conspiracy theory is that shaders were made consumable so that Bungie would make more money on microtransactions.
Best Answer
No, your exotic weapons will not lose the catalyst progression if you dismantle it. Myself and users on reddit have confirmed this.
To test, I used an exotic weapon that had a 3% catalyst progression as my guinea pig. I dismantled the exotic weapon and immediately acquired it through the collections menu. When I had viewed the weapons perks, the catalyst progression remained
The same is also true for exotic weapons with a completed catalyst and upgraded to masterwork. However, dismantling a masterwork exotic and then collecting it through the collections menu will reset the enemies defeated counter to zero.