To simplify my post above a bit. There are three rules:
1. Any account linked to a Switch can play any game purchased on any of the linked accounts.
2. If an account is shared to another Switch, the games on it can only be played by that one account.
3. A shared account can only play a game on one Switch at a time - and that Switch must have internet access.
What this means is if you have two Switches with 3 accounts linked to each, and one account shared:
Switch 1 has: Account A, Account B, and Account C linked to it; Switch 2 has Account D, E, and F linked to it.
Switch 2 has Account B shared with it (So Account B, D, E, and F are on it, but B is not linked.
If playing Stardew Valley on Switch 1 Account B, then Switch 2 CANNOT play Stardew Valley on Account B at the same time.
If playing Stardew Valley on Switch 1 with Account A or C (not B), then Switch 2 CAN play Stardew Valley from Account B at the same time
Again, hope this makes my above post clearer (or maybe it is even more clear if you just ignore my mess on my other post above! :P)
I describe myself as a little dose of toxic masculinity.