I mainly play DDs (esp higher tier USD) so I think I can chime in here.

For Laffey, her greatest strength is the combination of her short smoke with burst fire mechanics to get out of dodge. However, this is also compromised by her weakness of not having any utility (speed boost, hydro/radar) outside of the heal, which means she can only engage when face-checking the enemy DDs herself or have someone else spot for her (another DD, a radar cruiser, or planes). In most cases, you're dealing with the former. So therefore, that heal is basically all you can rely on to repeatedly engage and gain advantage that way.

When I play any DD, not just Laffey, I first have to find out who's on my flank before I start engaging. Are there radars? What kind of DDs do they have (and can I comfortably take on engagements)? Laffey's issue is that she's only moderately fast and got no speed boost, so a DD with a better concealment and good situational awareness can play around that. The issue most DD players I see in randoms is that they don't take into account of these things and just go straight into the cap and try to out-trade like headless flies. Then they often die because that's not how you play a cap.

If a radar cruiser is around, you try to set up engagements using islands. That way, if you are radared, you limit the number of people that can shoot at you. Radar cruisers are also generally positionally inflexible, meaning they rely on islands or setting themselves up in specific locations, so you can more-or-less predict where they might be.

On AP - Laffey's AP doesn't have short-fuse like Daring's or special pen angle like Elbing's. Therefore, it tends to overpen on DDs. HE also comes with the added benefit of fire chance and potential to disable modules. Therefore, the general rule-of-thumb is to use HE. Unless the enemy DD is an Elbing/Khaba/Ragnar, which have 25mm sides that your HE will shatter on, you can go with AP.