Well this makes interesting reading! I did not know you were rethinking formulas' existence in Free version, and you have seen my post elsewhere to the effect of I think you should indeed reconsider as they are so vital to any free "spreadsheet", so it is welcome.
I will say that I am not sure what your aim has been in dropping Rule.js in favour of re-implementing yourselves from scratch, especially in view of the amount of time it is taking and slipages.
Since I have had no choice but to stick with Hot 0.25.1 and Rule.js, I have made a number of bug fixes & enhancements to Hot's formula handling, still using Rule.js and without having had to change Rule.js code itself. While it has its problems, I have managed to fix bugs of incorrect cell recursive/forward references (which was the biggest problem area), validation, formula errors, caching, numeric typing, empty cell references, and more. so that now formula handling is acceptable. In that light, I wonder why you feel the need to abandon Rules.js, which is working for me? Of course, if you want sight of my code changes you only have to let me know!
[BTW, I have also made a small fix so that now Hot works fine on mobile, or at least iPad/iPhone! So I have formulas and mobiles :)]
I wish you a Happy Christmas!