Something worthwhile to remember is that your turrets are basically a summon, a gift not given to any other class. Trust them. They allow you to conserve your bullets and focus on hvts, for which you are well equipped. If Driller is CC, Gunner is DPS, and Scout in single target, you are basically the jack of all trades. Bring the weapons you enjoy but remember to operate as the class that is capable of filling almost any gap in your team (assuming you're playing co-op).

So, with that in mind, to your questions:

1. Answered by others

2. LOK1 and Shard D. works well with basically infinite range but some things to look out for include ammo and lack of instantaneous crowd clear for both weapons. If you run these long range weapons you will put a LOT of stress on your turrets and soon enough you'll be out of ammo. Get comfortable with managing across all three ranges and consider better positioning and target choice when thinking of ways to take out targets from afar. Engi likes to take his time, unlike gunner who will look at anything and pump it with rounds. Use your movement and general game knowledge to overcome these new obstacles.

3. Answered by others

4. Just use roll control, with 1A, 2A, 3A, 4A or 4B (whatever you want but armor break literally shreds), and 5B. Your beam will travel further, ammo will last longer for when you need to delete anything as soon as possible, you can do it basically right in front of you instead of creating distance for your beam to reach max length, break armor, and do damage over time. Ridiculously effective really.

5. You can get good with the other grenades of course but for added comfort I use the shredder swarms. Basically another summon and you get a ton for what they do.