It seems you've mostly figured this out now, but let's deconstruct a bit.
- Orphaned blocks are valid blocks.
- Orphaned blocks contains valid transactions.
- The transactions in an orphaned blocks can only be found in that orphaned block.
I think a better way to understand this would be to consider an orphaned block to have previously been a valid block, but no longer.
If the longest chain is at 300K, and there's e.g. an orphaned block 200,001, at this point it's almost totally irrelevant.
Any transactions included within it have almost certainly either been relocated into another block within the main chain, or simply replaced (inputs spent out from under them).
Consider that a transaction that exists only in orphaned blocks, but not in the main chain, is an unconfirmed transaction. If it never makes its' way into the main chain, it 'didn't happen', as far as the canonical state is concerned. The recipient, if they're aware of such, has good incentive to rebroadcast until the tx is confirmed again.
Aside from curiosity purposes, the only reason I can think of to worry about non-main-chain blocks is very close to the tip of the main chain - it's possible that they have not made it in to the main chain yet.