1) Is it a good idea to import the keys into another wallet and spend the bitcoins from there? (Knowing that there's still some unconfirmed transactions out there.)
Yes - to importing the keys elsewhere...
Maybe - to spending the coins... it may not be possible if the existing transactions are still in the mempool.
2) Can I cancel the unconfirmed transactions? With a lot of Googling, I could not come up with a trustworthy way of doing that.
No. This simply isn't possible with Bitcoin. You either wait for the transactions to drop from all the mempools... or they will get confirmed. There is no way for you to forcibly "cancel" them.
I'm following this.
I have similar question about 5.19 version. Is it even still working? Is it mandatory to update to newer version?
Technically, the app still "works"... however a lot of users have had issues with backups being corrupted (the encryption process seems broken)... and also, the fee system is VERY outdated... the MAX fee you can pay is nowhere close to current recommended fees.
There is no "newer" version... The entire MultiBit Classic and the more modern MultiBit HD have been dropped by the developer. There will be no more updates.
www.multibit.org My advice:
Export your private keys from MultiBit and sweep (or import) them into Electrum (
https://electrum.org/#download). It is probably the the closest wallet in terms of functionality and operation to MultiBit.