> Google findet über 10.000 Treffer für "handstanding", was zumindest auf eine real existierende ing-Form hindeutet.
If by 'real existierende ing-Form' you mean it can be a verb, then sorry, it doesn't mean that, at all.
I wish that people would be a great deal more careful about drawing conclusions from simple search engine hit counts. These can be very deceiving unless you examine the results carefully, and consider the actual query you have made.
First, there aren't 10,000 hits, there are really only 430. If you get to page 43 you will see that the remaining 9,570 hits, if they even exist at all, are copies of the first 430. You can find all sorts of Unworts that have many more than 430 hits. Try the misspelling 'seperate' and see how many you get.
So that's number one.
Number two, If you actually go look at some of the results you will find that they are gerunds--that is, nouns (Handstanding consisted of walking several feet..) and adjectives ("Lola the handstanding cat"). So most of the 430 we can throw out, as well.
To get a better fix on how often "to handstand" is actually used as a verb, pick a verb form that cannot be confused with a noun or adjective, like, for example, "handstood" and search for that. This has so few results (59 claimed, 33 non-duplicates) that Google offers a spell-correction for it.
To get an idea of how exceedingly rare 59/33 hits really is, take a word that no one in their wildest dreams would consider a verb in English, a word like "however", or "particular", or "something"--and "make a verb" out of them by adding "-ed" to the end, and see how many hits you get in Google. (Answer: howevered: 158/85; particulared: 173/81; somethinged:3,290/522).
Whatever the dictionaries may have to say about it, we can at least conclude that "handstood" is about 1/3 to 1/2 as common as "howevered" and "particulared", and 15 to 20 times less common than "somethinged". This is hardly evidence for existence of the verb form.
If the dictionaries include it, there must be a history of attestations in the past, but I agree with other native speakers who say they have never heard it, and whatever its prior use, it appears to be rare now (at least, in its past tense verb form, and it would be unusual to find a verb commonly used in the present that never occurs in the past).
The burden of proof is on those who support it to find evidence of existence, not the other way round (proving non-existence being, of course, an impossible task).