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My beliefs about roundie safety glass (I could be wrong):
1) The reason for the safety glass was to survive a certain strength of impact; it had nothing to do with making the faceplate stronger, but only in preventing it being struck directly
2) I believe the impact resistance had little or nothing to do with the bonded or non-bonded construction, that is the bonded roundies were just depending on the impact resistance of the safety glass the same as with a separate glass
If the above is correct, replacing the bonding material with air will have no significant effect on safety; it will mainly provide a place that could collect dirt.
3) The safety glass may have had something to do with decreasing the radius of scatter in case the tube was imploded, but I don't think so.
Later banded tubes did depend on the banding to strengthen the tube by applying pre-stressing compression. Removing the band from a banded tube is dangerous because it can allow expansive forces to develop under an impact that is too small to reverse the pre-compression when the band is intact. Glass is much weaker under expansion than under compression.
4) Color set safety glass I believe did have some lead content, which could contribute to reducing X-rays in addition to lead in the faceplate
5) The introduction of all-glass color tubes brought the possibility of X radiation off the back as well as the faceplate. Thus the inclusion of lead in the glass and warnings to technicians in some cases to operate with all shields in place.
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