Merging into a lane is a co-operative event, it is up to the road user wishing to move over to signal their intention and wait for a suitable space to move into. It matters nothing that the speeds are different, it is your responsibility as a following road user to be alert for other road users merging and adjust your speed accordingly, or move over to an adjacent lane (if one exists). Yes, if there isn't a safe space between you and the person in front and the person merging dives in (with or without signalling intent) then it is likely if an accident occurs they are at fault. This is no different no matter which type of road user is merging into traffic.
What happens a lot of the time is that people see a cyclist, bus, garbage truck, old person in a car signalling a merge and they speed up to close the gap. If there is a resultant collision it would at least partially be the fault of the road user who sped up in response to the signalling, as they have done the opposite to what is reasonable. If you are a Garbage Truck or a Bus, you pull over anyway (most of the time) as they are not intimidated, but others?
When I am riding in Toronto, I signal my intent to move to the left lane, to make a turn and I wait for a suitable gap in the traffic. You learn to judge the speed of the traffic and how far from the Intersection you need to make your lane change to do it safely and avoid annoying other road users unnecessarily. Yes, sometimes a vehicle will need to slow, but never in an abrupt fashion.
If we're going for the separate cycle lane approach, then the only way it can work safely is if cyclists heading straight on (in the cycle lane) have right of way over motorists making a turn. This is the case anyway, both with cyclists in the near side lane and pedestrians on a side walk. (Unless, in the case of the pedestrians, the cross walk is flashing before they step into road).
To solve the left turn is simple too, you show green to cyclists ahead of the regular traffic signal. They can then perform their left turn, whilst the traffic is held at red. This does mean the cyclists would have to wait for the appropriate phase in the cycle at the Intersection to make their left turn.
Cycling around cities in Denmark and Holland show that this infrastructure can be made to work (and retro-fitted to cities). Sadly, giving road space over to cyclists (at least in Toronto and I suspect a lot of other cities in the world) is politically unpalatable, even if it ultimately is the best for everyone.