It is assumed that ARM stepped into the fray as a legal obligation to the js [Toshiba & NEC] because of a patent indemnification clause

Agreed, because Toshiba and NEC had indemnification clauses. It appears that MEI did not have such clause.

They stepped up against the '584, and the '584 only.

Agreed, because Toshiba & NEC were accused of ARM '584 infringement, not ARM '336/'148 infringment..

If they had any (perceived) liability regarding the '148 or '336, they would have had no choice but to remain in the fray to defend their legal obligations on those patents as well

Again agreed, but since MEI had no such indem clause, ARM was not ask to join this litigation as in intervener re '336.. and probably (IKN) could not have enjoinjed re '336/'148 even if they had wanted to, since they are not obligated by any Indem clause covering MEI's ARM '336 accusation.. Can ARM just ask to intervene in this litigation re the '336 when they legally are not asked to.. perhaps legally can't be asked to (no clause)? Hope I am making myself semi clear..

SGE, thank you for the discussion.. you are helping me broaden & sharpen my thinking.. and that's tough duty :)