a) yes it is reachable in one day. I hitched Leipzig to Enschede (just behind the border) last week within 6 hours, though it can be much faster.
b) via Magdeburg.
My tipps would be like this: Get to the spot (see hitchwiki) and try to catch rides towards Halle/Saale at least, or Magdeburg. If only to Halle, get out at Halle-Zentrum, let the driver drive 100m on the city-highway (Berliner Strasse or so?) towards Halle and let you out at the first exit. There’s a bridge and you can hitch at the gas station on the other side. Worked pretty much for us, but a little bit tricky. Better would be towards MD, then, get out at the service station just behind Halle, if your driver doesn’t go onto the A2. A2 Rasthof Börde would be perfect, though.
From there, it’s easy to get westwards passing Hannover, most difficult part for me is always Bad Oeynhausen if you don’t already have a straight ride to Osnabrück or Netherlands. At Auetal service station you need to switch if driver don’t go onto the A30. By no circumstances get into Bad Oeynhausen, it’s a fucking trap. If anyway, the first service station (Shell) in the city is okay, though the owners don’t like hitchhikers. Best is at least till Melle, where the only A30 service station is located. There, catch anything to Holland. Nothing inbetween! First service station is 20km into the Netherlands at Hengelo, but from the second on-ramp in NL (Oldenzaal-Zuid) you can already start hitchhiking ramps (Auffahrt). It’s with lightning for night-hitching, pretty okay. The last km through the Netherlands are easy, 2-3 hours from the border till Amsterdam.
Wish you warm roads!
PS: There’s a hitchhiking meeting in Leipzig on 25-27 November, feel free to message me if you wawnt to know more!