Denis,
There is definitely something weird going on there, because I have several DFP2 scripts with 5 and sometimes 8 actions happening and I haven't run into any problems... do you get any more details from that Error 500? As far as limiting the input, the idea in DFP 2 is that you can resize anything you want going in so there wouldn't be any need.
Out of curiosity, are you on a dedicated host or shared hosting and might there be some limitation on the process time limitation or cache that is being controlled outside of your php.ini file?
