Why are there slab borders and building limits in general?
Slab borders are needed for house spacing, that’s why you must leave them, and the air above them, as they are. They are on two sides only, this is correct; you can build up to the edge of the other two sides. It’s suggested to fill the surroundings of the buildings with the same slabs because that's the city's pavement, in this way the builds are better integrated into the city layout :-)

What about water, is it allowed?
In some rare cases it may cause problems when houses are placed by the structure blocks; anyway, water is allowed. If water presents glitches in the rotated instances, it will probably glitch also in the final city! If possible. Try containing water pools only with solid blocks, and not with stairs or slabs only; they're the ones which glitch the most. If not possible, replace glitching water blocks with ice that it will melt down after your house is placed. If there are still some glitches or problems, they’ll be corrected before their integration in the Infinity Matrix.

Are we allowed to incorporate command blocks and redstone into our builds?
It's better to avoid complex circuits, while they should work fine, I'm afraid that if too many people build complex circuits that "do stuff" in their build, than this will make the machine much heavier to run on low spec PCs.
I don't want to risk that some people aren't able to generate their cities with your builds included because of this problem! If you want to do a special lighting system with redstone or a teleporting elevator with command blocks it’s ok, but try to stay contained and absolutely avoid automatic machines! Your devices must be user-activated!

What about the streets? How are they made? I’d like to make a parking lot or a building with road access…
The roads are 7 blocks wide, 3 blocks per lane and 1 for the centreline. Roads are made of the grey terracotta already present in the building sites, at the same level, pavements are made of the same stone slabs of the 2-side border, at its same level.
If you’re building something that needs a road access (e.g. like a parking lot) place the road-sized entrance/access aligned with the "entrance here" passage for 1x1, 2x1, 1x3 and 2x3 plots. For 1x2 and 2x2 instead, put it on the middle of the left or right chunk. In this way I can reprogram roads so that they connect correctly to your car entrances when your builds are placed in the city!

Are the items in chests, furnaces, dispensers, etc. saved in the Infinity Matrix too?
Yes.

Can I use paintings, banners, custom player heads etc.?
As a rule of thumb, if something appears correctly also in the rotated instances, then it can be used.
Paintings and item frames sadly can’t be used for now. Banners and custom player heads work flawlessly.

Does the front door have to function, or can it be an open glass door?
It doesn't have to necessarily be a door! It's just a constraint that sets where the front of the building is, so that the engine makes your builds facing towards the roads correctly. More like a "the door should be here" indication, intend it as "people will access this area arriving from here". After that you can put the entrance you want, of even something else! For example, in the largest plot, you could even put two small buildings on the left and right side and a small plaza in the centre, that would work too!

Will my builds appear together in the final city?
The city will contain as many submissions as possible from participants, so there will be houses/buildings from different people close to each other, a city from the PMC community!
Any generated town will be different and potentially infinite, in some of them your builds will be closer, and in others more distanced, and every building integrated in the Infinity Matrix will have a defined probability to appear during the generation Process. Consider all that is being created here as a single neighbourhood!

Did Gheppio design the procedural Infinity engine by himself?
Yes.