1). A proposed alternative to Segwit was to just increase the block size of BTC. The block size of BTC still remains at 1 MB. But at the time Segwit was introduced, come BTC developers saw the solution as simple as increasing the block size to 2 MB.
Anyone can see that that is a mere bandaid on the problem. Firstly, this would cause a hard fork in the chain and make previously invalid blocks valid. Secondly and more important IMHO is that this would only solve the problem for a short period of time. (Segwit has a time limit too IMO)
What happens when adoption increases and the 2 MB blocks now be come full, do we then hardfork again and make it 3 MB, 4 MB? We will reach these limits as adoption continues and do we just keep increasing the block size? Not the best solution.
2). Sewit solved transaction Malleability as well as the scaling issue.
By removing the Signatures from the input, we removed them from the hash and subsequently from the Transaction ID. Now you cannot go back and alter the TX ID by altering the signatures.
3). Sewit and the lightning network are connected as the lighting network is a second layer protocol. This boosts BTC’s transaction capacity by taking frequent, small transactions and moving them off chain. These transactions settle on chain once the users are ready.
4). No, people aren’t forced to use update. Adoption is slow.