I have only skimmed the full thread, so someone might already have said this; if so, I apologise.
Many people, me included, thought that a Labour government would be a very welcome change after 14 years of the appalling Tories. I am still glad that we have a Labour government, but very disappointed in the apparently frequent freebies, not to mention the threat of further austerity instead of some proper investment to grow the economy. Let alone the two-child cap and the WFP.
However - if RD is also disappointed in what has been happening so far, I think that she is right to resign the whip. Her constituents may well have voted for the LP manifesto, or to get rid of the Tories, or as a personal vote. If she is a believer in the previous values of the LP, including the manifesto, can't she still be representing her constituents as much as she did before? Therefore, no by-election required.