Stubby:
Looks like you have done this before. Even though I am a, old civil engineer, I have difficulty following your verbal description. Why not sketch what it looks like for pipe layout, T" fittings, etc. and take a photo or scan of it. You can add several of these sketches to a post and I think that will help a lot.
To simplify what you are proposing, and considering danger of working in a trench, I'd put together the piping above ground before the trench is extended down (cave-in is likely). Since your inlets will be slotted PVC, it is not necessary to fool with filter fabric if you surround the pipe with ASTM-C-33 fine aggregate. That is the coarse sand that is used in concrete mixes. That is a perfect filter for most soils. Placing the slots down also helps, but even if holes are drilled for the bottom third, 3/16" diam, that works pretty well with minimum of sand coming in.
What I do for under-drains, removing ground water, is as follows: Dig the trench and follow immediately with the black wrinkled slotted, pipe and backfill the zone near the pipe with that concrete sand. Do this rapidly before cave in starts. On these jobs I don't allow any gravel on the job, since well meaning workmen seem to think that is better. It isn't, since it is not a filter. For the slotted pipe very little sand gets in until the slots are bridged over. Concrete sand costs about the same as gravel and is plenty porous for these installations.
In this job, I'd backfill up with that sand to the water table, followed by the soil that came out. I doubt that a seal above the porous stuff will do any good at purifying the infiltration water, by diverting it some. I'd also be careful of any bentonite, since that stuff can expand to 16 times it's original volume and raise hell with the instillation.
On safety, I once was near buried in a 16 foot deep test pit trench in hard clay and since then don't enter any over 4 feet deep. For this job no trench entry is necessary. Just work fast.
An alternative would be to establish a "man-hole" with holes in the sides. Extend trenches out radially with drainage pipes (the slotted black stuff) backfilled with concrete sand. Put a sump-pump in the man-hole and you really have a source of water. The black plastic sump enclosures might work.