If it's "flattish", with only two or so "steps" like the 1986 law, then no, it shouldn't overly burden the indigent and people with marginal incomes.
Everybody should pay "something" .... and a flat tax would do that without crushing the poor.
2. How do you "abolish" the IRS if there are still ANY federal taxes to be collected?
Other federal agencies can be tasked with tax collection, e.g. Customs as in the 19th century. Payroll and W-4 taxes could be collected and remitted by Social Security or another Treasury agency.
3. If Obamacare is abolished, what - if anything - will replace it?
"Nothing" is the right answer. Private group arrangements and insurance will be more efficient, more effective, and freer of political interference and power-games than any federal program.
Obamacare was intended mostly to give employers a way to dump their employees' health plans and flow the premiums to the bottom line, ever so bonusably for the executive suite.
Companies that dump their plans can be made subject to punitive taxation, and executives' compensation bumps can be taxed away -- "legislating ag'in 'em" is an old American remedy for economic misbehavior.
Abolishing Obamacare is the realization of the basic idea that solutions closer to home than the District of Columbia with its welter of expensive entertainments are always better solutions. Unless you enjoy supporting Senator Menendez's sexual appetites ..... and other appetites and needs .... times 100.