It is fair to say that homogenizing (whether we are talking about miscibility or solubility) a nicotine/PG/VG mixture has its challenges. Particularly due to the viscosity of the VG and the oxidation sensitivity of the nicotine even at moderate temperatures. Adding flavoring or sweetener compounds only complicates this further.
It can be hard to tell if you have achieved nicotine solvation since it is a liquid with similar fluid properties to PG. but theoretically speaking, PG and Glycerol are both alcohols in which nicotine (which is not typically considered to be strongly polar, but does have a di-pole moment) can dissolve. achieving the correct conditions to do this in a DIY environment without degrading the nicotine or flavoring components of the mixture is what will give you trouble and that well known "peppery" taste reported by vapers when sampling mixtures containing nicotine oxides.
to respond to your other question regarding efficacy of a solution vs a suspension, nicotine suspensions are going to give you a pretty similar "end result" compared with a nicotine solution, assuming you mix it really thoroughly. Though I do not know at all how much bio availability data has been collected comparing the two. Considering the FDA does not regulate the industry anywhere near to the standards with which it regulates pharmaceutical manufacturing, I would say that the bio availability, degree of solvation, reported nicotine concentration of the mixtures and even the general safety of the e liquid etc varies wildly from maker to maker, be they "legit," DIY or otherwise. If an e liquid manufacturer doesn't sell DIY kits/ supplies and you cannot verify their credentials as a Biology or Chemistry degree holder, it is likely they are spinning something to try to protect their sales. If they do sell DIY kits, and are still telling you this, its probably because they know that after years of R&D that the quality of what products they are making in the lab is better quality than what you can make at home.
Disclaimer: I have consulted for e liquid companies, some that offer and some that do not offer DIY products.