<adding a bit in as I noticed something whilst typing this reply> As far as I know for the /target command it will land on the last valid target on the list, so you're macro should work off the focus target if you change the order to /target [target=party1target,harm,nodead][target=focustarget,harm,nodead]. That being said I still think your better off using a different method so you can free the alts target up for other things.

You seem to be using both focus and target to select which mob you want to attack (as you focus your leading toon and then assist them for targetting). Depending on group set-up this may not be the best method as basically it means you have very limited options when it comes to targetting a different unit (for example to use crowd control on them). Personally I think you're best sticking to casting spells on your focus' target and using the characters actual target for crowd control (e.g. fear).

So, first you need a way to change focus when you change to 'leading' on another character. I'm guessing Jamba does this for you, but I don't have any experience with that addon, so I'm just guessing. If not there should be a macro on the wiki for setting up one macro (which you'll need on all characters) to change the focus if the leader dies.

Your spell macros should then look like the following:

/cast [target=focustarget,harm,nodead] Lightning Bolt

This way your alts should cast the spell on whatever your focussed character has targetted, without them actually needing to target that unit themselves.