You pay pay btc.com with your VISA card, so that gets round your problem of having no confirmed bitcoins left to pay quickseller or macbook-air.
You insert your transaction and then click 'ESTIMATE PRICE', then you can pay with your VISA card so you can spend your bitcoin, the irony is unbelievable...
Quickseller was really nice and pushed my transaction without paying him upfront without questions (I'll pay him once the transaction confirms).
So, if I understood everything correctly then it's still possible that I have lost my money and my bitcoins will remain stuck in the middle of nowhere!? I'm not panicking but I'd like to know the worst case scenario! Is there any way to be able to get my bitcoins back?
No, it's not possible you have lost your money. Even if your transaction never confirms it doesn't get lost, the network eventually forgets unconfirmed transactions, and your Bitcoins reappear in the address you sent them from.
If quickseller's agreed to push your transaction I don't think you'll have long to wait for a confirmation. He got ahmedjadoon's transaction confirmed very quickly. You will only have a long wait if F2pool doesn't mine a block for hours.
Quickseller will push your transaction using F2pool, so when it mines another block your transaction will probably get confirmed. You can see the times it mined its latest blocks here.
https://blockchain.info/blocks/F2Pool
So basically the best I can do is to add the receiving address to my blockchain.info account's watchlist and wait till I get the notification email.
You don't need to bother, your transaction has already been pushed. You can see it at the site linked below (F2pool's transaction accelerator). Search for b1ce1b4d538b664cbae83fcdc34a5512f6bc2b7e48260bed7bba04553510b786 on that webpage, it's near the top.
https://www.f2pool.com/pushtx