So you are really asking a two separate questions:
Your second question is "should we implement a social engineering scheme to avoid the regressive nature of the sales tax"?
I say "no". We do that in Ontario and it has been such a nightmare because it distorts spending, complicates pst filing, and leads to people splitting up coffee orders at Tim Hortons into multiple bills (because a "meal" is tax as a luxury if it is above 4 dollars, and no, this was never adjusted for inflation and yes the tax applies to the whole $4.01 rather than just the cent) and other stupid things (never buy 3 muffins at Tim Hortons, you get bumped into the non-meal bracket, but buying 6 gets you the bulk discount AND exemption from the pst because your presumably buying 6 muffins to take home to your kids, rather than stuffing your face with a 3 muffin "luxury meal"). It is so stupid Ontario is doing away with it and harmonizing with the federal GST.
As for inheritance, this will just hit the lower class. Only people with enough money for lawyers will set up trust funds or "gift" their children $500k to buy their house for them. Also when you say "inheritance transfers at 99%" I assume you mean the government takes virtually all of it. If I were dying and had $100M I'd move to the Turks and Caicos and die there, where I can give my kids what I want.
As for your first question, which is basically "can we implement a simple VAT style tax (ie, tax both goods and services, but let companies deduct the VAT they paid against the VAT they collected) across everything and not greatly disadvantage the poor?" The answer is a resounding "Yes" except for a couple key conditions. First, give every adult a cheque for $7k a year if they have lived in the country for 10 months or more during that year. Don't ask for their income or anything. Then implement a 25% tax on EVERYTHING. "Ohh, but we want to encourage reading" NO! Don't let social engineering make this complicated. As soon as you bring in exemptions you bring in lobbyists, accountants, lawyers, and programming complications. All of which slow down productivity growth which, ultimately, reduces government revenue.
The second thing you need to do is issue a one time cheque to people that are retired, based on their current savings that are not in some form of non-income tax fund. The reason for this is that if someone has paid income tax their whole life and was planning on using their $200k cash they have in their bank account to live, you can't just increase their cost of living by 30% or they will starve.
That's it. Problem solved. No more tax forms, or telling the government how much you earned, or (besides the one time thing) how much you saved. No worrying about offshore bank accounts, or whether a freelancer should incorporate or not. More math brains in science and tech. More business brains in enterprise. The reason it won't ever happen? Politicians need to be busy bees.