Benjamin Trial NP2 with Center Point 3-9X40AO

Hello all. Im kinda new in Airguns, if we exclude time i was a kid and had up to 200 fps basic airgun.
As my first more serious airgun - Benjamin Trial NP2 with Center Point 3-9X40AO - which is possible to buy with out permission ( in Poland ), this was the strongest on market up to 17 joule, a bit better FPS than Gamo Whisper IGT. So my decision was to take Benjamin Trial NP2.

This sunday I was on range and had very good time, even if it was windy, I was able to put pallet into pallet at 15 yards, than I moved to 30 yards and next to wind i was able to put pallets into black cyrcle at 5.5 inch targets. Than happen something unexpected.

I was in a prone position and doing break barrel action and on some 70-80% it barrel slip out of my hand and my airgun shake.
Well how strong Im not able to say, but im sure im not the first who did it... it happens.
After this I load next one ( Exact 4.5 mm ) and shoot. I was not even able to hit cartoon box of HP PC on which I attach many targets.
So i was afraid that my Gun lost range, than i shoot at some sand from 3 m range and I saw its going 1m higher than I aim.
And than I shoot next one at 15 yard cartoon but aiming over scope, not using crosshairs and I hit it. So i understood that my paralax is to high, and I decreased it as much as I could and still on my last test - about 5 yards I was aiming bottom of BLACK cyrcle and my shots landed 1 inch higher than top of BLACK cyrcle on the same 5.5 inch targets.

Ok, my supplier is big Polish company and importer of Crossman and many other brands, they will take it under warranty.
But I have next questions:

1. Why it happend and is this Center Point 3-9X40AO so bad as basic scope, or it would apply to some more expensive scopes?
2. Could my Benjamin Trial NP2 cause some damage, and drop FPS ?
3. What would be the most optimal scope for this kind of rifle, what you can recommand ?
Im thinknif of two examples from link below, its what I have availible for not so big money and i guess that 50-70 yards is limit for this my Benjamin Trial NP2 0.17

a) cheaper version - http://www.hawkeoptics.com/vantage-3-9x40-ao-mil-dot.html
b) more expensive - http://www.hawkeoptics.com/vantage-side-focus-4-16x44-mil-dot.html

Could you please give some advice ?

Thanks