The 2 inches shaved off the antenna shouldn't have a significant impact on your receiver's performance unless you fly your Slo-V near maximum range. You can actually just solder the piece you cut off back on if you wish, it will work fine.

As for your Slo-V climbing at full throttle, that's what it's supposed to do. The wings are designed as "undercambered airfoils" and they provide a great deal of lift, even at low speed. You will need to use the throttle as well as the elevator to control the altitude of your Slo-V. While this doesn't make the Slo-V as manuverable as other designs, it is part of why it's such a great slow flyer and a great first airplane.

Congratulations on your Christmas present! You should have a lot of fun flying the Slo-V!