grammar - In/on for "into/onto" in colloquial and not so formal AmE

If the context is crystal clear and, as such, allows no risk of misunderstanding or ambiguity whatsoever, unlike "Paul jumps into the lake (= Paul jumps into the lake from a certain point)" vs. "Paul jumps in the lake (= Paul is in the lake and jumps out)", is it grammatically acceptable in AmE to substitute "in" and "on" for "into" and "onto" to indicate movement or the idea of movement, for all but the most formal prose?

E.g.

Should we go in the pool first, or should we shower first? source

Let's get on the bus before the good seats are taken. source

Let me translate this in the language of non-BS... source

Answer

In and into are not, strictly speaking, grammatically interchangeable in formal writing. On and onto have a little more leeway, but are also not always interchangeable.

But, colloquially we often use in for into.

It is perfectly acceptable to say go in the pool, jump in the pool, etc. But, as you correctly stated above, that introduces ambiguity:

I am going around in the pool. -- Unlikely to be the intended meaning, but it could be.
I am going into the pool. -- The more likely intended meaning.
I am urinating in the pool -- This might also be a likely interpretation from the colloquial usage of go to mean urinate (Thanks Digital Chris!)

I am jumping around inside of the pool. -- Again, unlikely from context.
I am jumping into the pool. Most likely scenario.

You can get on the bus, and you can get in the bus. But, they have subtly different meanings:

Get on the bus - Board the bus with the intention of riding.
Get onto the bus - Is awkward since it implies climb on top of. Better to say get on to the bus.

Get in the bus - Get inside the bus for whatever reason.
Get into the bus - Get inside the bus for whatever reason.

But, note that we don't get on the car. We do get in or into the car, though.

For whatever reason buses, trains, planes, and other large conveyances have a notion of being board-able (employing the preposition on), but cars and vans generally do not. (This may be convention.)

Compare this to a bicycle, motorcycle, etc.. You get on a bicycle, but this is the physical sense of being on top of it.

Languages are always translated into one another. Never in. But, you can speak in a language.

Some other cases where on and onto are not interchangeable:

Turn on the lights.
Turn onto the lights - You'd probably never say this. It implies that you physically are turning yourself onto some lights.

But, they are interchangeable when on carries the sense of on top of.

Put that on the counter.
Put that onto the counter.