what worries me is where they say about cross chaining, I'm not sure if that would break the chain but would probably make pedalling feel hard.
I avoid cross-chaining too, but it really doesn't matter. It puts minimal additional stress on the chain, it's quite likely that there may be a bit of rub (and therefore noise) on the front derailleur cage - use this as a catalyst for changing to a better chainring/sprocket combo; and the loss of efficiency is (according to a paper that seemed based on experimentation/testing/research of good quality) at most about half a per cent.
Edited to add: For a given ratio, using the large chainring and a large (even the largest) sprocket (say 50/25) is more efficient (eg 11t > 21t = +3%) than using a smaller chain ring and a smaller sprocket (say 34/17). This advantage dwarfes the cross-chaining issue (from an
efficiency PoV).
"Human Power" Number 50 Spring 2000 $5.50 TECHNICAL JOURNAL OF THE IHPVA
"the effect of chain offset can be estimated.
These data were obtained with
the 52–11 and 52–21 configurations in
the offset condition while those in the
long-duration tests were taken with no
offset. Comparing the data for 60 RPM
100 W tests shows that the offset lowers
the efficiency by, at most, 0.5%
when measurement precision is considered.
Additionally, if the efficiencies are
normalized by efficiencies measured in
the 52–15 configuration (both sets of
data were obtained with no offset),
then it appears that the offset has a
negligible effect on efficiency."