Nothing in life or politics is guaranteed.
True but the act of parliament for phase 1 has passed and contracts awarded. Only people who know little about the topic or are totally against HS2 think there is any real chance of cancellation.
Lines that would require a subsidy are not likely to be funded. Most of the schemes that are likely to happen are ones that would generate larger premiums for the government e.g. East West Rail or at least not cost a fortune every year e.g. Portishead.