1.) The main psychoactive constituents of psychedelic mushrooms are psilocybin and psilocin (a few varieties also contain baeocystin and norbaeocystin, but we can ignore these for now). Psilocin is a tryptamine with two methyl groups attached to the nitrogen atom and a hydroxy group attached at the 4-position (which pharmacologically is rather strange, as usually compounds with an exposed hydroxy group cannot cross the blood-brain barrier). It would be written out 4-hydroxy-N,N-dimethyltryptamine, or if you prefer Shulin's abbreviations 4-HO-DMT. Psilocybin is the phosphate ester of psilocin and acts only as a prodrug (i.e. is not active until the body has metabolized it into an active substance, in this case psilocin). So really for our purposes psilocin is the compound of interest. Depending on where and how you modified the psilocybin molecule the effects wcan and will vary greatly. For example if instead of two methyl groups attached to the N-position you substituted two ethyl groups you would have 4-HO-DET, a compound with dosage, duration, and effects that seem rather similar to those of psilocin. If however you were to substitute in two isopropyl groups in place of the methyl groups, the resulting comound, 4-HO-DiPT, has effects quite different than those of psilocin. The dosage is around the same, however it is very intense as well as much shorter, with a uniquely short duration of only 2-3 hour, something which is very unusual in psychedelic world.
2. I'm no expert of the law, but if you were to make any kind of substituted tryptamine and use, sell, or promote it as a psychoactive compound, you could be prosecuted under the Federal Analog Act which states that:
(A) Except as provided in subparagraph (C), the term controlled substance analogue means a substance -
* (i) the chemical structure of which is substantially similar to the chemical structure of a controlled substance in schedule I or II;
* (ii) which has a stimulant, depressant, or hallucinogenic effect on the central nervous system that is substantially similar to or greater than the stimulant, depressant, or hallucinogenic effect on the central nervous system of a controlled substance in schedule I or II; or
* (iii) with respect to a particular person, which such person represents or intends to have a stimulant, depressant, or hallucinogenic effect on the central nervous system that is substantially similar to or greater than the stimulant, depressant, or hallucinogenic effect on the central nervous system of a controlled substance in schedule I or II.
Substantially similar brings up an interesting debate as to what exactly such a vague wording means, but there is certainly no doubt that even if the substance you created was not specifically scheduled as an illegal drug, you could easily be prosecuted under this act.
If you are interested in the study of psychedelics, do check out TiHKAL and PiHKAL by Alexander and Ann Shulgin, they are extremely interesting books both on the cultural and scintific level.
Hope I managed to clear up a few of you questions.