WebSetBrowser() only sets some internal behaviors for the replay engine. It's been a long time since I discussed the details but as I recall it wasn't much. I recall things like the way parallel TCP connections are used and possibly the User-Agent header being affected by WebSetBrowser.
In no case does a Web Business Transaction(HTML/HTTP) recording make any use of any actual browser for playback. That recording type is driven purely by the replay engine in perfRun.exe.
Have you tried recording with Firefox? I can't recall when support for that was added but I think it's there in TM ART late releases.
Browser "age out" is a significant issue with TM ART and one I seriously doubt will be fixed. BMC has moved on to the new TrueSight Synthetic Transaction product.
South Coast Software Consulting