The Intent Exclusive locks trying to convert to exclusive aren't your direct issue here. They'll attempt to escalate at 5000 rows but it's not required, it's just an optimization to keep lock usage down to reasonable volumes.

Can you confirm that you get TableA back as the object in the following query, please? By preference, please simply post the results, but I realize you've modified things slightly to obfuscate certain things... well, unless you actually HAVE a table called TableA.

SELECT * FROM sys.sysobjects where [id] = 21575115

Also please reconfirm TableA has no indexes whatsoever, also check for unique constraints and anything else that might be hanging around.

By rights, an Update and an Insert should not collide in a non-indexed heap in any way. Inserts will tail insert and updates will work against existing data pages. Even splits shouldn't affect it. Something is odd here.