Flushing only needs to be done in 3 events IMO, when you over do it on the nutes or the PH gets out of hand. When you switch from Veg to flower and the final flush before harvest.

Droopy plants normally mean too little or too much water, they aren't getting enough O2 to the roots or they really need a drink. Find out which one it is and slowly adjust your watering schedule to the plants needs + or -.

When you flush your plants use pure clean water and to speed up the process use flora clean or a flushing product, it weakens the bond the nutes have with the plants allowing them to be removed easier.

As long as your PH is ok I don't think the pops hurt the plant...its mostly sugar and water. They don't look all that bad really, they are still really small so take it easy on the nutes until the 5th or so growth node, then kick it up.

Just be patient and your plants will tell you what they need, you will probably do more harm being overattentive to everything. They need consistency, good water, nutes, ventilation, proper lighting and the right temp./humid...most overlooked is ventilation, its vital to get good growth and for the plant to strengthen itself.

Is this some sort of drip/DWC system?

Keep your res temps 68 or so also...key point in DWC. I wouldn't even screw with the drip part just keep the airstones going and they will splash the right amount of water of the bottom of the pots.

You don't need to change out a res that size very often at all....plants that size didn't even make a dent in the nute solution, the reason for changing out the res is that the plants take in nutes and dump out waste, this process will make your solution unbalanced so monitor by PPM, PH, and what your plants look like....I might even keep them in the same nute solution for 3-4 weeks it just depends on how much they are eating.

You don't have to cover the hydroton...it would be better though, less light to the roots and nutes = no algae and better root development.