Simply extract whichever .ymt file(s) you want to use from that mod into lml/stream. If there isn't a stream subfolder, create one first. This is explained briefly in the readme within the Lenny's Mod Loader archive. The 'replace' and 'stream' subfolder functions work perfectly fine, it's only that the mods installed in this manner will not show up in the mod manager GUI.
There are 3 versions available. If the latest doesn't work, try the older versions and see for yourself🤷♂️. No one is obligated to advise you on finding a working version of a mod for a pirated copy of a previous build of the game. If you want to play and mod a cracked version of the game, fine by me, but I'd say the burden of testing and problem-solving is on you.
They are not online-only weapons, in fact from what I understand the m1899 is not available in RDO at all. So yes, you need to progress in the story to unlock them.
With WhyEm's, most of the new content is added at the trapper, with its own submenu added to the main store menu. Impossible to miss if it's there, although if RDOffline isn't working then I'd wager WhyEm's isn't either.
Texture resolution and screen/game resolution are completely separate and independent of each other. Your screen and game are running in 4k but doesn't affect the textures, only the texture quality setting does. Not sure the standard res for Arthur's face on ultra but I'd imagine 2k. Texture resolution differences are easier to distinguish on higher resolution screens though so it would actually make more of a difference on a 4k screen, not less.
Been experiencing regular "Red Dead Redemption 2 has stopped working" freeze-crashing after 10-30 minutes of playing (usually within the first 20) while using beta 8. I created an account here to report that today I played for between 90 minutes to 2 hours or so blissfully with no crashes and quit the game normally after reverting to beta 7. I know one play session not a very big or convincing dataset but the crashes were so consistent before that I'm optimistic. But I intend to report back if anything changes.