A series of enormous windfalls.... and some questions

[barrelomonkeys]

We had some enormous windfalls this year:

-DH got a substantial grant to fund his research for the next 2-3 years
-DH's income jumped from $47k to $100k in October, making our Gross Income for the year ~$63k
-A different company we held a substantial portion of got bought out, making capital gains of ~$270k ($170k in our taxable account, $100k in a DH's Roth)

Other information:
-Husband is the only paid employee of his company (it has a CEO being paid in stock), but DH is technically not "self-employed," so we cannot do a solo 401k :(.  DH is going to hire someone in ~6 months.
-Started out setting up benefits too late in the year, so we missed the deadline to set up a SIMPLE IRA
-DH Contributed $2,104 to Roth this year, I contributed $1,749 to Roth this year, so we'll need to fill out and submit an excess contribution form to Vanguard by tax time

I keep seeing conflicting information, so here are my questions:
-It's super late in the year, but can we set up a regular old 401k and somehow put in the max $18,500? Or does it have to come through a payroll deduction?  If not, I think we'll either do a SIMPLE or a 401k next year.
-Will the $18,500 actually reduce our overall AGI of $230k?  I don't know why I can't find a straight answer to this.
-As far as I can tell, a tIRA wouldn't do us any good, really, since it wouldn't actually reduce our taxable income, it would just shield it from being taxed while it grows (which it isn't doing right now anyway).  Any other reason to do this?
-Anything else I can do before the end of the year?

Thanks in advance for any help in this arena. I have really appreciated the collective wisdom of this forum in the past, so thanks :).

[fell-like-rain]

1. 401(k) plans must be funded through payroll deductions. There's no way to add money otherwise or do "catch-up" payments. If you set a plan up right now, you could contribute up to 100% of any paychecks left before the end of the year, but that's it.

2. To my best knowledge, 401(k) contributions do decrease your AGI. I think the confusion comes because tIRAs are an above-the-line deduction, whereas 401(k) contributions actually decrease your earned income- so if you made 50k and put 10k into a 401(k) and 5k into an IRA, your earned income would be 40k and AGI would be 35k. (Of course, you still pay FICA taxes on 401(k) contributions, but that's another thing)

3. You pay no income tax on tIRA contributions (UNLESS you have access to a 401(k) and are above the income limits), then you pay regular income tax on withdrawals. The gain comes from a. avoiding capital gains tax and b. the income taxes you defer were at your top marginal rate, whereas the taxes you pay in retirement are at your average rate (i.e. some gets shielded by deductions, some is taxed at 10%, then 12% and so on)

[barrelomonkeys]

Thank you for such a detailed, thorough response.  I really appreciate it!

[fell-like-rain]

No problem- happy to help.

[Valvore]

I like going through older posts. I'm curious on what you ended up doing @barrelomonkeys ? And congrats on the HUGE bump in your income.