Rulings
Q1: My opponent blocks BT3-058 BanchoStingmon. Do I still get its effect off?
Yes, you still get BanchoStingmon's effect. Blocker happens after all When Attacking effects and BanchoStingmon's effect keeps the changes 'for the turn'. (See more in Attack Resolution)
Q2: I attack my opponent's Digimon with 12000 DP or more with this Digimon, then one of my effects reduces that Digimon's DP to be under 12000. Does this Digimon's When Attacking effect activate?
Yes, it does. This effect behaves the same way as an effect that reads "When this Digimon attacks one of your opponent's Digimon with 12000 DP or more". As such, the requirement for 12000 DP or more is checked at the time the attack is declared and determines whether or not the effect triggers. After the effect has triggered, this requirement does not impact the effect's activation.
Q3: I have activated BT1-104 Golden Ripper and attack with BT3-058 BanchoStingmon. If I attack with this card and use BT1-104 Golden Ripper's effect to decrease the attack of the attack target from 12000 DP to 10000 DP before I use its effect, can I use this card's effect to give it 7000 DP?
Yes, you can still use BanchoStingmon's effect if the attack target's DP decreases before BanchoStingmon's effect activates as long as it was 12000 DP when it was targeted for the attack.
Q4: If this card attacks a Digimon with less than 12000 DP and then one of my effects increases it to 12000 DP or more afterwards. Can I use this effect?
No, this card's effect did not fulfill its trigger condition.
Data Sources
Lv.6 BanchoStingmon BT3-058 U Mega | Free | Insectoid/Boss <Piercing> (When this Digimon attacks and deletes an opponent's Digimon and survives the battle, it performs any security checks it normally would.)[When Attacking] If you attack an opponent's Digimon that has 12000 DP or more, this Digimon gets +7000 DP and <Security Attack +2> (This Digimon checks 2 additional security cards) for the turn.
Lv.6 BanchoStingmon BT3-058 U Mega | Free | Insectoid/Boss <Piercing> (When this Digimon attacks and deletes an opponent's Digimon and survives the battle, it performs any security checks it normally would.)[When Attacking] If you attack an opponent's Digimon that has 12000 DP or more, this Digimon gets +7000 DP and <Security Attack +2> (This Digimon checks 2 additional security cards) for the turn.
Lv.6 BanchoStingmon BT3-058 U Mega | Free | Insectoid/Boss <Piercing> (When this Digimon attacks and deletes an opponent's Digimon, it performs any security checks it normally would.) [When Attacking] When this Digimon attacks one of your opponent's Digimon with 12000 DP or more, this Digimon gets +7000 DP and <Security A. +2> for the turn.
Lv.6 バンチョースティングモン BT3-058 U 究極体 | フリー | 昆虫型/バンチョー ≪貫通≫（このデジモンがアタックしたバトルで相手のデジモンだけを消滅させたとき、このデジモンはセキュリティをチェックする） 【アタック時】DP12000以上の相手のデジモンにアタックしたとき、このターンの間、このデジモンのDPを+7000し、≪セキュリティアタック+2≫（このデジモンがチェックするセキュリティの枚数+2）を得る。