It's a contradiction in terms for an "undemocratic" manifesto to win support. Either the people support it, in which case it's democratic, or they don't, in large enough numbers, in which case better luck next time.
In practice the LibDems will not win a majority, firstly because not enough Remain voters will switch their votes -- you kind of need everyone to believe it's worth it at once, and I don't think they do -- and secondly because even if a fair few did, the system is not really designed with LibDems in mind. In a reasonable* scenario with, say, Corbyn's vote collapsing in favour of the Libdems, so that the national vote share is something like LD/Con/Lab = 35/30/25, then the Tories are the largest party in seats, and despite a massive 10-point lead Labour are only about 20 seats short of the LibDems.
So a lot of things need to go right for the LibDems even to make any kind of significant impression. I'd be surprised if they got more than 50 seats and amazed if they got more than 60.
*I use "reasonable" in the loosest possible sense here.