Why have schools decided that it's OK for boys to use the girls changing room if they declare that they are girls really, and push girls who are unhappy getting undressed in front of their male peers into using the toilets instead?
Apart from they're not. This is copied directly from the Mermaids trans inclusion toolkit:
Any pupil or student who has a need or desire for increased privacy, regardless of the underlying reason, should be provided with a reasonable alternative changing area, such as the use of a private area (eg a nearby toilet stall with a door, an area separated by a curtain, or a nearby office), or with a separate time to change (e.g. using the changing room that corresponds to their gender identity before or after other students).
Why do MPs shrug off concerns from women as simply being nasty bigots?
Because a lot of misinformation is being spread. See above.
Why do big companies suddenly start letting men into changing rooms previously segregated by sex?
This has been legally happening since the GRA was passed in 2004.
Why does Twitter ban GC feminists who say things like "women don't have dicks" but take no action against TRAs who threaten violence and rape?
Back to the TRAs again, I have no idea. They are both pretty unsavoury and should both be banned imo.
Why does the Labour party take disciplinary action against GC feminists but allow schemes designed to correct the current imbalance between male and female representation to be accessed by men?
If you are referring to Lily Madigan she was democratically elected.
Why does the Girl Guides accept boys, and suggest that girls shoudl leave the organisation if they don't like it?
Probably the same reasons the scouts accepts girls. Inclusion.