I try to answer a few of your questions:
I don’t understand exactly, what you are talking about. Testing while developing or checking spelling etc. before putting to air?
You can test templates while developing by installing a CasparCG server on your developer machine and play the template out to see, if it works.
You can add a another channel to your CasparCG server, that you use to play the templates to check spelling etc.
It is not possible to play, for instance, a full screen PNG sequence, without stuttering. So if you have a area that is fully transparent, you can reduce the size of the png-sequence. Typically lower-thirds and the like only uses an area near the bottom of the frame. Make the sequence as small as possible and position it on the HTML page, where you need it.
See this tread for a good tutorial.