Your crafting skill is only used to determine the probability of success. It doesn't destroy the weapon if you fail, nor does it allow you to make better weapons. The chance of failure is low at the start but goes up substantially at higher levels. For example, a level 60 character with no crafting skill creates weapons with the same damage bonuses as a level 60 with crafting. However, the level 60 with no skill better have plenty of gems to spare, because it wouldn't be unusual to use 5 gems before one succeeds.
As for adding additional gems, there's a limit. It varies depending on the power of the original enchantment. I believe the maximum is 5 extra gems. At low levels, you might only be able to add 1 extra gem. I describe the weapon "on hit" level elsewhere in this forum. Each extra gem adds to the "on hit" level. The progression is 1d8, 2d8, 2d12, 2d12 plus 1d8 of extra "on hit" damage, 2d12 plus 2d8 of extra "on hit" damage, etc. The "on hit" level is the total number of d8s from the base and "on hit" damage dice, with 2d12 being counted as 3d8.