When doing the 128 mb mod i first place the chips in place the using smd rework hot air i get them tacked down. Then i go over each pin with a soldering iron. then i clean it up by going over them with hot air again to make the solder look nice and clean. Im sure you can get it correct with just the hot air , but so far i have not had luck with that. I need to get better it seems. but that works for me.
From personal experience with flashing a tsop with a bad image, a modchip is much safer to test bios images on. If you somehow brick a modchip (unlikely) you can just replace it. with a bad tsop flash you can swap the tsop from another console of the same revision.