I still have a bit of time, less than a day on my Extend Storage 1 and I wanted to extend it by another 3 months, so I bought a ticket and when I use it well... do I have to wait for it to expire before using the 90 day Ticket?

Yes, you have to wait for it to expire. The only thing that can stack longer than 90 days is Premium, and that is 180 days.

Yes, and don't worry your items wont disappear

In fact you can still take stuff out of expired expanded storage, you just can't put anything in it.