In a situation where the Soviet Union reaches the Channel (by any scenario) would it be possible for them to pull of Sealion in any form?
No. They posessed no naval force of note to pull it through with any more success then Nazi may hope.
And reaching the Channel in almost any scenario means very exhausted Red Army.

There one interesting twist though - if soviets reach the Channel in TL where there already was a successful Sealion and Britain is occupied by Nazi.
No, under say the normal OTL with POD after 1920s the Soviet Union will be unable to invade Britain once it reaches to channel
Without any special events, once Soviets will be able to build up their invasion force to succesfully attack Britain, the British might have nukes.

Of course another questions is if the Soviets can effectively destroy Britain by invading Middle East and India.
That depends on how long you accept between the red army reaching the channel and Sealion attempt.

If the red army is at Calais in 1944, it's likely the USSR has a chance of a successful Sealion by 1975 ( or maybe slightly earlier ), given that equipment devlopment, training and doctrine won't be anything remotely like OTL.
If the Soviets get to the channel Britain would quickly become very xenophobic and reclusive and nationalists would likely get more seats in government, its likely they will be even closer to the United States and you may very well see the UK in Vietnam. Also things like the EU probably won't develop the same, the Anglosphere will be the bulk of western democratic power since all the other colonial powers will be soviet influenced.
IOTL the Soviets had plans for an invasion of Britain during the Cold War.

They would take Blackpool and a town (I forget which) on the East coast so that they could cut Britain in two.