It's not bad. I imagine that would have been close to the contemporary marks of Hurrican, Spit, and 109 (a little lower but close). Plenty of stuff later in the war would have been able to beat that though.
Yes, the Mosquito would have been ideal for taking down bombers. There altitude wouldn't have been a problem, and four 20mm was more ideal than 8 .303.
The Me-110 was good but it was totally outclassed by escort fighters (even the P-47). A truly effective interceptor needs to have a reasonable chance in a fight with the enemy's escorts.
Not if you're using British tactics Lightning, with Hurricanes for the bombers, Spitfires for the fighters.
Me-110s for the bombers, 109s or 190s for the fighters...
Well the T-bolts could take the formations to the German border or so and the Lightnings could pick it up from there. The main problem was that there were only 2 P-38 groups in the 8th AF and they could usually only sortie about 50 planes to cover a few hundred bombers against a couple hundred German fighters.