Airline credit and change fee

I recently bought a departure and return ticket from NY To Tampa and I had to cancel my flights. I got a credit of 239 for one ticket and the other I got a credit of 291. Now both tickets are with different airlines so the change fee is 150 for one and 200 for the other. How do I make the most of my credit now for when I want to book another flight (any destination). I just want to make this as cost efficient as possible.

Going to Germany for 77 days and can I go to UK for 2 months, and enter Germany to take my return plane?

I'm non EU national and my passport allows me to go to Germany for 90 days without a visa and 6 months to go to UK without a visa.
My plans are to learn German for 75 days (it is an intensive course) BUT because it is a expensive long haul flight, after it, I want to go to UK to stay with my friends for 2 months and travel using ryanair. As my return ticket would be from Germany, can I enter Germany just to take my return flight to my home country? (I'm afraid as the entrance is for 90 days only). Thank you very much.

Added (1). I mean, travel Europe using ryanair.

When will I have to go thru customs and immigration on this plane trip?

Crazy, but the cheapest way is USA to Canada to Iceland to Amsterdam to Abu Dhabi to my final destination of Kuwait. I will not be visiting any of those countries, just staying in the airport changing planes except AbuDhabi I stay on the same plane that goes on to Kuwait. I'm a US citizen. Thanks- never had such a crazy trip before, but need to do it cheap! Will I have to claim my luggage and go thru all that mess in every country or can I check it thru when I get on the 1st plane in the USA and then pick it up and do all that in Kuwait?

Should I sue an airline for the money for destroying my laptop?

Qatar Airways security prior to boarding (inside Hammad International Airport) waved the metal detector wand directly over my laptop after asking me to take it out of the case. My bare laptop was then opened and had the metal detector wand scan each surface of it. I trusted them since they knew more than me about what and how they needed to search but after the security my laptop was not functional at all, yet it was perfectly fine before this happened. What should I do?