I'm still using up my bottle of Loctite 603, which was the predecessor of Loctite 648. Looking at the data sheets, 601 has a slightly lower strength, 22.5MPa compared to 25MPa, but it cures faster, so I'm not planning on changing until I have to.
I've only had to remove cranks/wheels from the axles of a loco in one case. Predictably it was the only loco with outside cranks fixed with Loctite.......
I didn't need to use any heat to dismantle the cranks, but I carefully cleaned the crank seat on the axles with solvent before remaking the joint.
As you say, make up a test joint and see how much effort it takes to dismantle it.