Repair or Replace Shifters Two fold question. MTB has 7 speed cog and 3 chain-rings. Shimano Deore LX rear derailleur and Acera front derailleur with rapid fire shifters.

First, the right shifter will now only index four cogs. I've checked and adjusted the rear derailleur and even when cable is detached the shifter will only advance or descend four clicks. Is it repairable?

Second, can I replace the rapid fire shifters with grip shifters?
You can try saturating the shifter internals with wd-40, which may help. You're not going to find replacement parts, so they're typically not repairable. 

7 speed shifters from Microshift or in Shimano's Tourney line or SRAM's Attack! line are compatible, so I'd suggest just spending the 10-20 bucks and getting one of those instead. The SRAMs are grip shifts, as are some of the Tourney shifters. The SRAM ones are probably a bit better quality (note the Attack! name; regular SRAM shifters will not work). The Microshift shifters and Shimano Tourney shifters will be easiest to find. 

That being said, my 7 speed bike had the shifters go after higher quality than the bargain basement Shimano shifters weren't so easy to find (altus/acera+). I ended up moving to 8 speed and getting a good quality shifter.