Happy to help in general terms here.
Generally speaking, risk of audit and high fines and fees go up as the amounts you're dealing with go up. I don't know what kinds of values we're dealing with with 40-100 trades, so I'll just give you broad strokes. Having an accountant or a tax attorney review your numbers will, in almost all cases, cost you less than $1,000. If you're talking about $2,000 in income/gains, then obviously that's a non-starter and I'd just go the route you're considering (SaaS), having done good due diligence on the service as you seem to have. If, however, you're looking at $100k in OI or capital gains, I'd say a 1% investment to not owe all of your back taxes + potentially 100% or more in fines in 5-7 years when, potentially, you don't even have that money anymore.
As for your second question, I would just say there is no statute of limitations on undeclared income. The logic there is that you aren't incentivized to hide your income for 3 (or 7) years and then get off the hook, and it is difficult for the IRS or state taxing authorities to discover undeclared (as against under-declared) income -- i.e. how would they know about it? Thus, there is no amount of time that can pass such that you are truly free and clear.
You can file amended returns for ordinary income you failed to declare with Form 1040X: https://www.irs.gov/pub/irs-pdf/f1040x.pdf