1- it's the same 3D object. but black arcade and white arcade have their feet black and white, so they don't share the same pillars. However the mid pillars are the same 3D objects, but our editor doesn't support that kind of complex genealogy. so you have:
Black Arcade that place Black Arcade Pillar
>Black Arcade Pillar<<=insert Tech Path Straight here
>Black Arcade Pillar feet
White Arcade that place White Arcade Pillar
>White Arcade Pillar<<=insert Tech Path Straight here
>White Arcade Pillar feet
I'm not fan of this either, but we do with what we have.
If the Mid Pillars had black&white components, you wouldn't be puzzled
2- I could make that version, but soon you'll ask for the 90° ones

Sometimes I prefer to switch to other more usefull blocks rather than make the complete set. see... I didn't have enough time to make the pillars in water for tech towers, and I think they are really missing.
3- can't you place the tower core pillars above, they will open the tower?? I forgot to test it after changing the system of the core pillars to be more suited for hooks. I'll check that and debug if necessary.
4- what do you mean by "fit" ? 3.3 are tech path (high regen rate) and 3.5 are bunkers (nucleus)
5- these (TechTower) are the first pillars I made for the tech path while searching for the right grammar.
Tech path were not supposed to fly, like castle carches, and be sticked to a pillar.
The first time I've put a techpath corner into a tech tower I discovered these pillars were too big and would ask complexe modelisation for every block I would put into them
I changed for the half ones (TechTowerDeadend), that worked a bit more like the arch pillars (I planned to convert them to arch pillar and win all the terrain possibilities of these).
To not loose the benefits of turning the tower 90° over itself (and then allowing crossing path above each other), I made a switch from the tower to the half tower, up and down.
That was giving a sci-fi WTF look.
Is it a space ship, a power plant, a giant alien screwdriver ?
Anyway, whatever funny name you give them, they were puting the storm environment far from earth (and then far from uncanny valley and far from AK47 or M4A1).
If I had that time in my hands, I would change the screwdriver head design (I already started it, but it's lower priority than pillars in water I think) to a more "conventionnal" one that will last longer. Maybe you'll see alternative tops for these towers one day ;p
To come back to the question : you can place any block in a halfTower pillar, convert to a tower, turn 90° then convert back to a half tower and then put another block 90°.
It takes a lot of space because the transtions are 4 blocks high, and so they are a bit useless since we have flying blocks now, but they also give a lot of "weight" to the towers.
But they still make funny Sci-Fi structures. and maybe one day may be able to support very heavy Tech structures, who knows?