Your English is miles ahead of my German, so don't be sorry.
1. Structures have very heavy armor. You are correct. Long range frigates (LRM, Illum, Assailants) don't get a bonus, but they aren't bad at it. They do 75% of their damage to structures, which isn't bad.
2. I'm not sure what you mean by defence platforms. If you mean turrets, they do no damage to SC. They do anti-medium damage.
If you mean the hangars (not sure if one of them is called the defence platform or not)
TEC Hangar: You can buy autocannon's for them. They do 40 dps per platform.
Advent Hangar: They can get 750 shields and regenerate 4 shield per second on it and every building around them.
Vasari Hangar: They can phase out enemy strikecraft for long periods of time. No damage done, but enemy loses use of those for some time.
3. SB have very heavy armor.....i think. I know bombers destroy them effectively, so I think they must have very heavy armor. I don't have access to the game right now, but my guess is Starbases do either starbase damage, or capital ship damage. I think starbases do 100% of their damage...regardless of the target. That's a guess.
4. You are correct. Be careful though. Light frigates take time to destroy carriers. You should use some flak too when going up against carriers. Those strikecraft can target stuff in the grav well after the carrier is dead.
Example: Enemy has 5 advent carriers (15 squads)
I'm TEC. I'll get 15 flak (1 flak per squadron), and then 8 cobalts OR 10 LRMS. I say 10 LRMS because once you destroy the strikecraft, you can jump in LRMS and they CAN chase down the carriers. Carriers are now slower than LRMS, and LRMS are much more useful than cobalts overall.
Cobalts WILL destroy the carrier faster than LRMS. They do anti-heavy damage, as you said. LRMS, however, will destroy anything else the enemy brings, cobalts most likely will not.