You might consider a dry start - this will give the MC time to establish roots & lots of leaf growth
Without CO2, you can still expect leaf “melt” after flooding, but the plants should be better able to recover
If planting & flooding same day, I’d leave some small distance (maybe 1cm) between each “clump”
If you have established filter media, I’d add shrimp (2-3 days later) to clear away any leaf melt
Clithon diadema snails are also excellent (note they are more delicate than other nerites)
Try boiling the wood - pick up a cheap pot as some wood can release a dark tarry substance that’s virtually impossible to remove (without industrial grade solvents) - this will usually speed the sinking process
If wood remains stubbornly buoyant, then a slate base may be the best solution
If wood begins to sink, then a rock placed on top of wood in aquarium (until wood finally sinks) usually works ... I’d leave the rock in place for several weeks