definition and usage for whipsaw?

Your reference gives a premodifier usage it wrongly terms adjectival, but which is certainly relevant.

Here are two examples of the premodifier usage given at the Merriam-Webster entry for 'whipsaw' (though it does not give the recent metaphorical sense for the noun):

1. The whipsaw nature of the state’s regulations ... [Scott Wilson, Washington Post, "California reverses reopening as coronavirus cases spike," 30 June 2020]
2. The whipsaw action came just one day after a historic rout that saw the blue-chip index drop by 2,013 points, the most ever. NBC News, "Stock markets rally, with Dow surging 1,000 points at closing bell," 10 Mar. 2020]

The metaphorical premodifier usage is now common currency.

And further recent examples collected by M-W:

3. One described the effect as a whipsaw, saying the company and union would try to convince workers to accept a bad deal or see jobs shipped away. [Eric D. Lawrence, Detroit Free Press... 16 Nov. 2019]
4. The president’s rhetorical whipsaw came against the backdrop of tense but cordial meetings in Biarritz, France. [Michael D. Shear, New York Times ... 25 Aug. 2019]
5. The whipsaw of weather and market forces make for long seasons of uncertainty.[Autumn Schoolman, Indianapolis Star, ... 4 Feb. 2020]

show that the metaphorical pure noun usage is also gaining currency. Also, the 'president' example shows that there is a broadening from the 'dilemma' problem sense or 'involving successive ups and downs' problem sense to the 'speech highlighting the dilemma' or 'speech involving a series of U-turns') senses, one of which you ask about. However, 'rhetorical whipsaw' is probably less jarring than a plain 'whipsaw' would be here.

I'd be careful about using the bare noun as a metaphor for a whipsaw/dilemma-related speech or argument. Especially when there is more than one unbroadened metaphorical usage of 'whipsaw'.

The nounal usages correspond exactly to the verbal counterparts:

[A] whipsaw: to beset or victimize in two opposite ways at once, by a two-phase operation, or by the collusive action of two opponents

• wage earners were whipsawed by inflation and high taxes

[Merriam-Webster]

[B] whipsaw: ... To [move or] cause to move or alternate rapidly in contrasting directions: The bond market ... continues to be whipsawed by fears of rekindled inflation (Steven E. Levingston).

[AHD]

So is "He delivered a whipsaw [lecture]" a lecture about a two-pronged assault we need to counter, or one full of U-turns? I'm not even sure which senses are intended in most of examples 1-5 above.

It can be used figuratively as a noun, but it tends to have a slightly more specific sense.

To "whipsaw" is as you say a verb meaning "figurative (U.S. slang) to have or get the advantage of thoroughly, to overcome completely, ‘cut up’." (OED)

When used as a noun it has a slightly more specific meaning "figurative. Something that is disadvantageous in two ways. Originally and chiefly U.S." (OED again)

A whip-saw or whipsaw is a long saw operated by two people, one at each end. It's easy to see how this gives rise to the sense of to "cut up" metaphorically, i.e. to defeat or demolish in an argument or in negotiations. But also having one person at each end gives a more specifical metaphorical sense, something like being destroyed by two people working together from opposite sides. The OED's examples for the noun are all used in this specific sense ("The wage push..and the rising interest rates..have together caught the American economy in a cruel and sharp whipsaw" 1967; "the whipsaw effect of recession and rising costs" 1977; "There was fifteen hundred on the turn—seven hundred and fifty on each side of it—and the run was tray, ace; a whipsaw." 1873).

It's possible for an argument to attack from two sides, e.g. by presenting two alternatives, neither desirable, but most arguments would not have this specific structure.

"whip-saw, n.". OED Online. September 2020. Oxford University Press. https://www-oed-com.nls.idm.oclc.org/view/Entry/228443 (accessed September 14, 2020).

https://en.wikipedia.org/wiki/Whipsaw