Control Font Weight Expression

It sounds like you're already aware that you can't mix weights/styles within a single text layer if it's part of a Mogrt template. 

The best solution I've found for this limitation, which is doing essentially the same thing in a less code-y way:

• Create a duplicate of your original text layer, and set the style for this new one as desired - bold, in your case. 
• Pickwhip the Source Text property of your Bold layer to the same property on the original text layer. 
• Bold layer: Add a text animator for Opacity. Set Advanced > Units to Index. Set Advanced > Based On to Words. Now use the Start and End to adjust the words visible as Bold. 
  • Add these Start & End controls to the EGP, or connect them to sliders that will be added to the EGP. 
• Copy/paste this text animator to the original text layer. Set Advanced > Mode to Subtract. Use the Property pickwhip to create basic expressions linking the Start/End properties and anything else you find relevant. 
• Whether the spacing is awful or passable depends heavily on the typeface you're using. I've typically found that adding another text animator (probably to the Bold version) for Tracking, and adjusting by 1-2 units is usually enough to make this work well enough. Depending on what else is happening in the template, giving the user the ability to make slight Position adjustments to the Bold version can also be helpful. 
• If you need multiple instances of Bold within a single text block, you can add a second text animator (to both layers) to achieve this. 

While the above method isn't 100% perfect, I've used variants of this to create many highly-functional templates for many clients. Confirming exactly how the template will need to function, how long the text entries will typically be, etc. should usually allow you to design something that is functional enough without needing to over-engineer it. 

Suggestions for when the above doesn't quite get you there:

• Add a Tracking property directly to your Opacity animators. Bold will typically require slightly more space than lighter weights, but you can compensate by specifically adjusting the Tracking only for the words being bolded.
• If you need more font/weight options, you may want to actually want to create a "dummy" text layer that exists specifically to receive the user input from the template, which is then passed on to all your visible text layers. This opens you up to taking advantage of the text styling expressions, though that's a whole new can of worms!

Let me know if you need more direction on any of the above! Text animators are extremely powerful once you wrap your head around the (very different) way they work. 

Then I guess you'll have to indeed run through the whole rigmarole of figuring out the indices and splicing stuff. Have you tried contacting the original author from that linked thread?

Mylenium

It seems you're making this unnecessarily complicated. If the layers are separate, you don't need any advanced expressions and can just link the font selection directly. Calculating the line wrapping is a whole different story, but that won't work reliably with paragraph text, anyway, given that the Adobe Type Engine does its own magic internally. You have to use conventional line text. Therefore it might help to actually know what the texts are supposed to be rather than throwing around lorem ipsum. In fact if you used a monospaced font you wouldn't even need convoluted calculations for the positioning, just manipulate the text string.

Mylenium