I just saw the latest episode. I admit, I almost started crying when William said he wanted a Green Arrow action figure. The same goes for Oliver's entire message to William. These shows might be cheesy at times, but they sure can hit you right in the feels.

Ah so they are using "slingshotting" concept since there is no speed cannon in that universe (They did something similar to get the Reverse Flash back to his time period earlier in the season with the Two of them running in tandom to create a rift and the Reverse Flash getting a boost from The Flash to pass through)

Have to wait and see, they may overlook that (but again we don't know what Supergirl's top speed is), or in the comics The Flash has an ability to temporarily transfer speed from one person to another.  In the comics Jay Garrick used this ability to take the speed from Superman and Black Adam which allowed him to time travel.  Or they could just use the Silver age explanation where a speedster can just vibrate there molecules to the same frequency as another dimension which allows them to travel to that dimension (they did this all the time in the fifties and sixties) the issue with this is that it means Zoom would be able to do that as well making the dimensional rifts they sealed completely pointless