You will be claiming from KLM, not from Delta. Buy essentials. Keep all receipts - ensure you get receipts. You can claim reimbursement after the fact, but only if you can provide a clear legible receipt for each claimed purchase. You will have to pony up the money yourself first.

Buy what you need, under the circumstances. You can make new purchases each day that you still haven't been reunited with your bag. KLM will not refuse any reasonable receipts. Don't view this as an opportunity to buy a new, high-end wardrobe for example.

(If you're going to be buying non-essential items, souvenirs, etc, make sure to buy these in a completely separate transaction. Make sure when you are buying essentials for which you wish to be reimbursed, then buy these together, without any frivolous or questionable purchases listed on the receipt).

You can claim for these essential purchases here. It's not going to be an immediate reimbursement, so my advice to you would be just to keep the receipts carefully, and claim for all essential purchases in one go after you get home.

There is still every chance that the bags will show up. Bags are not usually deemed lost until 21 days after you have checked them in. Even then, the liability of the airline is limited by the Montreal Convention (unless, in certain cases, where you specifically drew the airline's attention to a particular item of value in your luggage). If the contents were worth more than that limit, then it would be up to your travel insurance to make good the loss.

It is NEVER a good idea to check breakable or difficult-to-replace items in checked luggage. All glasses and contacts should be carried on your person or in your hand luggage.

I'm not sure what you mean by a "surface", but if it's a laptop, that should definitely be in your carry-on luggage, too. Never check electronic equipment in checked luggage; checked baggage can be thrown, fall off carts, have other items fall upon it, or end up in a bin under 20-30 other bags and have all that weight pressed upon it for the duration of along haul flight. And - as you've seen) - the bag can simply go missing. You should never put breakable or valuable items in your checked luggage.

It's ALWAYS a good idea to pack as if your luggage will be delayed upon arrival by at least 24 hours. But always pack the essential items - housekeys, medication, wallets, credit cards, certificates, currency, electronic items - in your carry-on luggage and keep them with you at all times.