If you're gonna repack the front bearings, they need to come out to do a proper job.
Yes, you 'can' pack the cap with fresh grease, but it won't do a thing for the inner bearing, and it things get too crowded in there it'll start to blow grease out the inner seal which in turn may eventually spatter up onto the inside of the rotor and wreck the inner pad.
Second on the list is take the cap off, the nut, and the outer bearing and repack the space in the center so that when you put the outer bearing back in, fresh grease is squished into both the inner and outer bearing.
I'd not suggest either method actually unless you are prepared to take some risks.
Better to take it apart proper, clean both bearings (great way to inspect them too

) and the hub, repack everything, put on a new seal (ding-ding-ding-ding!! Expendable item!! Don't save it, replace it!) and head off down the highway in the confidence that you did it right onnaconna seized or cooked wheel bearings are pretty high on the suckatude list AFA dealing with it after the damage is done. (wreck 'em good enough and you'll be removing the inner race from the spindle with a torch or cutoff wheel. And that's not taking into consideration the possibilty of the spindle and/or hub being scored up badly before the bearing finally craps totally)
Oh, just to clarify: the left hub has left-hand threads on the spindle nut, so you might be a while if you try the lefty-loosey-righty-tighty thing with it.
On that bolt that can walk back into the hub?
If you ever do a full pulldown on the hub, you can fit one of those snap clip thingies onto the outside of the bolt to prevent it from slipping back into the hub ever again. (dunno the name for it, but it's like a flat spring steel washer w/three 'prongs' in it that you press straight on and the prongs hold it in place)
T'won't hurt a thing as the only thing that the bolt in question holds
is the bracket on the hardline. (others in that area are for the dust shield)