Speaking to pt no. 4 first - I second what @rigs has said, you should join a training alliance. There are lots of training alliances that would help you develop your hero roster efficiently. Many new players start levelling their 5* heros not realising they cost heaps in mats - Good thing you’ve been reading information on the forum.
Also, defeated titans drop Ascension materials you will eventually need to level up any 4*+ heros.
Def team, I’d suggest keeping your team 1 but from left to right
Belith, Bane, jahangir, Karill & Baltz - Jahangir is slow but seems the most tanky of the 3* you have already levelled. Belith has the additional buff and healing, both Bane & Baltz fast, although Karill being average might not get to fire tucked into right wing.
Raiding - try stacking 3/2 with the strong colour vs the tank, then double up on the strongest flank.
Heros to ignore
Depends on your own game play 3 star heroes, Keep vs Trash
And… Slow mana heros do very well in very fast 3* tournaments. I’d also keep copies of fast mana heros as you need 30 heros for war. They are very handy for clean ups.
Good luck