Halftrack does 32mph - it would not be second to the M18 - as 99% would still use the T34/85 for its protection and better gun - it would still get used over the M3 - only because its gun is a horrible piece of crap - however its probably at the bottom of the GV use.
T-34/85 is perked, although not heavily. And its not much harder to see and kill than any other tank.
The SdKfz 251/22 on the other hand, is about as fast, smaller, harder to see, and its free. Aside from the M18, its probably going to be the best vehicle to piss off a Tiger II with.
Another note - the Sd.Kfz. 251/17 is the standard Anti aircraft variant of the Sdkfz - it had a single 20mm cannon - later on it had a single 37mm mounted gun - the Sd.Kfz. 251/21 was introduced in August of '44 for the need of Anti aircraft vehicles very few were made.
And so? Didn't only one squadron of F4U-4's see service? IIRC, weren't there less than 50 ostwinds made?
The Ostwind and Wirblewind didn't go into production much earlier, with the first Mobelwagen (the predecessor of the two) only going into service in April of 1944.
If anything gets added it would be the single 20mm or 37mm version - why? Wirb/Osti were in production when the triple mount was made - allies need a few anti aircraft vehicles first before axis gets any more beefed up then they already are.
So HTC would intentionally leave a vehicle update unfinished, simply because the Allies don't have something to match it, despite all sides having access to it in the MA's?
As for adding the earlier single 20mm or 37mm gun, wouldn't that only exacerbate the issue in special events, considering that the Axis would have a more effective AA platform even earlier than they do now?
Hell, when you actually think about it, adding the single 20mm option would actually be dumber than adding neither.