hi! hmm i'm not sure why using a css class won't work for you, however if you can share the full template and find a way to replicate the problem in the playground i can try to find the reason.
about doing a line break you can just put a <br /> and put a overflow: scroll style in the cell with the line break (live example). this works fine in latest version jsreport-html-to-xlsx@2.3.0. you can try to install it manually or you can wait until we release jsreport 2.4.0 which is almost ready for release.