Just standard table cleaning
Yes. Although it does depend on what exactly you mean by standard table cleaning :-) plus there's a caveat with shellac1.
Some level of this should always be step one in a refinishing project if the intent is not to strip (and sometimes worth doing even if you are going to strip) regardless if the piece is not visibly dirty or grimy. Dollars to doughnuts when you're done the cleaning liquid will be greyish, which tells you that you removed something, even if it's only some dust.
After drying repeat the process with clean water if you think it needs it.
Leave to dry for an hour or more before proceeding.
Wash with denatured alcohol until the scrub rags don't darken appreciably
No, not if the piece is indeed finished in shellac.
As already highlighted in the Comments by @Ashlar, alcohol is the standard solvent for shellac so you'd want to be very careful or avoid it entirely. Since there's no benefit to using it if you follow the below advice I'd avoid it entirely.
Consider very light application of scraper even though I suspect the top is veneer
You can, but I think you probably won't want to given the risk. Shellac is already a thin finish, despite appearances, and even light scraping with a single-edged razor runs the risk of exposing bare wood.
It is wise to be cautious about going through veneer whenever we encounter it, but a lot of vintage and older furniture can actually withstand a certain amount of power sanding (!) as can be seen in many MCM furniture restoration videos on YouTube. Which is to say, where it's appropriate to scrape you can probably get away with it in many/most instances2.
Additionally, if the table is finished in shellac, which was already obsolete as a commercial finish before WW2, the veneer is likely not to be the paper-thin stuff we're now too familiar with :-)
Other
Again if a piece is old enough to have shellac as its finish there is a very good chance there is at least some trace of wax polish on the surface because this used to be standard practice and only fell out of favour in recent decades.
Wax removal
Any wax present needs to be thoroughly cleaned off the surface prior to the application of any subsequent top-up finish (even if it is more shellac).
So after your careful standard cleaning you should wipe down with mineral spirits (UK: white spirit) and plenty of paper towels or shop rags – the idea being that you are dissolving and then picking up any wax present, then taking it off, not merely spreading it around as you would if you use the same paper towel/rag for too long.
Note that there are commercial wax polish removers on the market, but their active ingredient(s) can be principally or entirely some equivalent of MS.
Last step
To ensure adhesion the surface should be made matt/dull, so that you're following the best advice for new finish over old: clean and dull.
1 Shellac can respond poorly to water, so it's best never to get it really wet. As a result it's best to only use a cloth dampened in the warm, soapy water, work one area at a time and immediately wipe dry before moving on to the next area. For a table of medium size doing it in quarters is probably the way to go – whatever is covered by a comfortable sweep of your arm.
2 Pun intended, scraping is far less of a blunt instrument than sanding and is to be preferred over sanding nearly always.
