Closing a channel is really simple. Go to the 'Channels' tab, right-click on the channel you want to close and select 'Close channel'. You should use 'Force-close channel' only if the other party is offline. In such a case, you will get your funds back after up to two weeks (2016 blocks). Most channels can be force-closed with a 144 block (~1 day) delay. In both cases, you won't be able to specify the closing transaction fee. Other implementations have such a feature.
Are these block numbers also arbitrarily chosen by Electrum or are they part of the LN protocol? It doesn't make sense to me why parties need to wait for 144 or 2016 blocks to get back their funds. These are real mainnet blocks, and the funds are in a multisig wallet, so by my intuition the funds should arrive in the first block that contains the closing transaction.
Also, do these numbers only apply to force-closed channels? So in that case 144 blocks is the usual delay and 2016 is the maximum, right?