that sounds like an excellent idea to back down on the primary drive control -- yes RV1. your signal should be cleaner if you can get the 40m output down to say 7 watts or less. most power meters (even the WM2 I use) can't be trusted to extreme accuracy. a factor of 2 in output power is about one half a S-unit -- so better to be more clean.
I am very surprised to blow the fuse into a (hopefully) 50 ohm load. possibly a higher current fuse is encouraged. I don't myself see danger is using a fuse that is 2X the expected current. most of the time I use a ~ 3 amp power supply to feed by ubitx. not sure what fuse Sunil supplies but that is what I have in place.
there is a greatly diminishing return on output power versus DC supplied -- if you ever operate using battery - less drive via RV1 is highly preferred - with little loss on the receive end.