Is there any way to get returning player content on PC without previously playing?

If a reward or unlock is marked as any form of exclusive, that is the only legitimate way to unlock it. That is why it is called an exclusive unlock, and not just an unlock.

I would normally tackle a problem like this by ticking off what needs to be done on console at a friends house; However, when I looked it up I found the content is not simply unlocked by playing the game on the console. You unlock the content through your character progression, and when you transfer your character over, the exclusives unlock. The transfer is a once per character offer and does not allow you to transfer from PC, Xbox One or PS4.

TLDR: You can not, legitimately. Even if you unlock it properly, you will need to do so on a new character

The Native Trainer included with Script Hook V allows you to spawn the vehicles and weapons exclusive to the 'returning player' content.

Once it is installed, press F4 in-game to show its menu. To make it give you weapons, go to 'Weapon' » 'Get all weapons'. You will then have all available weapons, including the railgun and hatchet in your inventory. To make it spawn a vehicle, go to 'Vehicle' » 'Car Spawner' and then choose the vehicle you want to spawn.

According to its readme, it won't work on GTA Online.

Script hook closes the game when player goes Online, this is done because the game reports installed mods list to R* while being in Online mode.

This is presumably also to avoid getting the player banned if they have the Script Hook installed and are attempting to play multiplayer. So it'll only work in singleplayer and you'd have to remove it when going online.