Sounds like a 50 quilt will be perfect for you. At those temp ratings the difference between down and synthetic is not that great so you might also consider an apex filled one from EE for example. Although, if you are really strapped for space while on your bike I guess the down is still that last bit smaller and lighter.
I don't think a liner will add that much warmth, but it might make sleep better, (I know I like sheets even if it is blazing hot in the night). I'm now considering just a bug bivy or bug inner, with my clothes - perhaps bring a puffy jacket for when it does cool down. But I'm talking about 15~20˚C (60~70F).