As you pointed out they're not exclusive alternatives. Given that launching Barbarossa with Britain still in the war and the US supporting both countries is 100% guaranteed to lose it, then Britain
must be dealt with first. Compromise peace rejected in JUN40 so invasion as attempt to knock it out of war. Failure = need to win by other means
I'd make the odds on US intervention much lower. Isolationist feeling was still strong, Lend-Lease was only narrowly passed and Roosevelt was very careful not to go ahead of public opinion. It's actually in my mind unclear as to whether he actually wanted to take the US to war or prefer simply to bleed both Britain and Nazi Germany. And remember he still didn't seek a DOW after Pearl Harbour.
Why should Britain make peace after losing the war in the Mediterranean and facing strangulation of its trade routes?
Err, It's
losing. Its domination of the middle east will be loosened after Cairo and the Suez canal are seized. I'd expect pro-Axis revolts in Iraq to succeed, Persia to turn hostile. Its grip on India is shaky. And Japan has occupied French Indo-China and is threatening Malaya and the NEI.
Unless the US is willing to join the war openly at this point, it will occur to a lot of High Tory's and others in the UK that peace now, however humiliating, will be better than fighting on and losing the Empire. Remember that in June 1940 some cabinet members (Halifax at least, with Butler as his deputy) were prepared (or so some reports have it) to seek Mussolini's intervention even if that meant giving up Malta and Gibraltar.
So Churchill being ousted in this desperate position (possibly beforehand once disaster looked certain) and a peace-minded administration being formed seems highly plausible.
So this strategy is plausible. Maybe 50-50 is too high, but only if you think the US will intervene actively.
A failed Sealion that led to a significant weakening of the RN would be a mixed blessing. Damage to the RN good but offset by morale boost to Britain. So I'd not go ahead with the full sealion.
OTOH, an early "air lion" might be a very different proposition.
FWIW I found this in
The Narrow Margin To paraphrase
"On June 4th, General Milch flew over Dunkirk, surveying the relicts of the evacuation and the material left behind. It was clear that the British army had abandoned nearly all its heavy weaponry. [...] On 18 June he proposed to goering that all available paratroopers and air landing forces should be despatched immediately to seize airfields like Manston and Hawkinge. They would be reinforced by regular troops in follow on waves"
'The plan involved considerable risks, but it might have succeeded' was the verdict of the book's authors.
So I think "Airlion" doesn't quite fall under the ban that the unspeakable pinniped does.
You, of course, are entitled to disagree. As are others here.
Is there an ATL with a mid-late June 1940 air-led invasion on the board? If so. I'd like to read it
I can see all the problems but really both Fighter Command and the British army were at a very low ebb in mid-June. Only one organised Division (Canadian0 and probably fewer than 400 operational Spitfires and Hurricanes (331 at June 5th from the Narrow Margin again).
The psychological shock alone could well have pushed Churchill out of office.