It's early days still very much a work in progress and A learning curve for me but what I've found so far supports both your ideas.
I started with ten tall plants , cut most of them into sections just long enough to get a good inch or so stem to bury in the sand. Some grew taller within a matter of days / first week.
Absolutely you will get new separate plants.
Some of those first cuttings when they floated up accidentally had good new root growth starting.
It has been pain staking , at the start everything just grew upward , that was a bit disapointing.
I re cut them quite agresively when they were tall enough to make two plants , I read that the more you cut them and the closer you plant them together the more they compact and stay low ( see the plants on the left of that photo , the new growth when planted close has smaller more compact leaves and they have much closer inter nodes too ).
So maybe it is working. The idea of the thread is to test the lights with that plant specifically instead of assumung the PAR values of a $12 Lamp and $3.50 bulb are very high just because the theory says it should be....
So I will keep taking pics every couple of weeks.
As for the tied down one , I read a lot of people just take those mother plants and bury the heads, seems to work so far.
I envisage that the first month or two will involve lots of cutting and replanting to force the plants to carpet.
If I had my time over I Would plant the first batch as closely together as physically possible and not worry about spread initially, Just have a 'bush' that I add onto over time / let the plant grow out too.
In a perfect world the shop ones would be super compact when sold.