#3 – I believe what you are asking for is dual gauge HO/HOn30 (aka. HOn2 1/2, or OO9 or HOe) track. The outside railes are spaced 16.5mm for HO/OO scale and the inside rail is spaced 9mm for HOn30 (30 inch gauge in HO) with one rail in common between the two gauges. 9mm happens to also be the gauge for N scale standard gauge, so in theory you could run N scale equipment on the same track. Because the scales are different HO (1:87) and N (1:160), the ties and rail would be greatly oversized for N Scale. However, if that doesn’t bother you, then go ahead and try it. Switches (turnouts) may be a potential problem as the gap in rails at the frog may be too large for the N scale wheels. Also, I believe that N scale is operated at a slightly lower voltage than HO scale. I believe Peco sells the HO/HOn30 dual gauge track. Fast Tracks sells assembly jigs that you could use to make your own track if you wish to go that route. Micro Engineering (ME) used to sell dual gauge Flex Track, but ME track is not presently available – perhaps you may find some on ebay.

#4 – TT scale (1:120) never caught on in North America as it did in Europe. It was overshadowed by the popularity of HO and N scales. You can find North American equipment available in TT scale, but it is scarce as there are few manufacturers (due to popularity of HO,N). Today TT scale is mostly a scratchbuilder’s scale for North American modelers. Try googling “TT scale American trains” and see what comes up.

#5 – Yes. 33 actual feet = 1 scale mile in N scale (1/160): 5280 / 160 = 33

#5 33 feet equals 1 mile in N-scale.