1) if you can't make it to 15, then don't force it. The normal version of the armor is pretty useless unless its the +versions.
2) go for the big 4 armors (Atlantean avenger+, hydra hunter+, crius+ and living flame+) these armors are the ones you should be going for. At least 3 of them will be a good idea to have a good armor foundation for later stages.
3) That's a quite okay pace I would say
4) Go for training fields If you don't play often because they have a decent gold per hour rate and can hold quite a large sum of gold.
5) at your level, there's really not much point to buy gems unless you want to use it to upgrade your castle. When you're higher leveled, you can consider opening the dark prince chest to get nemesis armors for bossing
By the way, I wouldn't recommend fusing armors since you're still low leveled.