The alternatives to increase Waterloo capacity if CR2 is not built, are all well described in the NR Wessex route study (2015), it covers both scenarios on an either or basis. As I've posted before they don't do both CR2 and 5-tracking, it is one or the other.
Can be quickly summarised as:
infrastructure works to provide a 5 track main fast railway from Hampton Court Junction (Surbiton) to Waterloo. It would operate with two main fast lines in the peak flow direction, using a reversible centre track of three. Can apparently be built within the existing NR boundary. Requirement at Wimbledon to remove the trams and move Thameslink over to the other side of the island.
This allows more trains to stay on the fast lines, and therefore more stoppers to run on the slow lines.
Then 12 car platforms on all main suburban routes, with major work required at Waterloo in addition to what is happening this summer.
There's far too much to put in a single post, but most of this subject has been covered in previous threads anyway.
https://16cbgt3sbwr8204sf92da3xxc5m...2016/11/Wessex-Route-Study-Final-210815-1.pdf