Sounds like you have your technique down. The rest of the questions about what will happen when you test can only be answered when you take the test. Will they allow you to backweld after removing the backing bar. Probably not. When they allow backwelding, it is usually w/o a backing bar.
Can you use a grinder? Dunno. Some allow it some dont. You just have to ask the tester. Running 7018 if you have your heat right, the flux should just fall off. If it is hard to remove, you heat is not right. If the flux falls off then simple brushing is all that is required.
Weaving is again up to the tester and the specificaitons. Somp allow you to weave others limit the weave to 1.5 times the rod diameter.
Good luck you are going to make it. Maybe not on your first try, but keep trying.
BABRT's
On your horizontal test, when you are putting on the the cap, try and keep the rod angle 10-15degrees above horizontal and the same angle away from direction of travel. This will tie-in the preceding bead better imo. As was mentioned, it is up to the tester as to whether you can use a grinder or not. Best to be prepared to test without one. You are allowed surface crack no longer than 1/8" on any one sample, this would be the total of more than one crack in any sample. As far as welding, you should stick to stringers, but ask the tester about using a weave on the verticle test....they might prefer that. You sound like you know what is going on, so keep it clean and you'll do fine.
Be well,
Hawk