Should be pretty easy to replace the battery on it, there are screws and everything. I've only had it 7 months or so....but i suppose it depends on how long its been sitting on the shelf...............
Yes, my Polar occasionally suffers from interference. Don't know from which source. In towns it can go wild, but suffers less in rural areas.
I'm sure Polar would claim it's interference free, but this is not my experience.
Send it back to them. I tried to replace the battery myself thinking that the waterproofing didn't matter on a bike. I was wrong. Had to buy a new HRM in the end as it got condensation inside. There customer service wasn't great (took ages to respond, never answered the phone) but they sorted me out with a discounted replacement. I had read that there customer service was now much better.
I only ever got two years out of mine. Should be due another one soon. Have to say on both occasions I have used Polar Customer Service they have been very quick in responding and they have the watch and strap returned within a few days. Can't complain. Guess I must be the lucky one. I had to get both the straps replaced on my hrm because the wrist strap broke. I thought it was a bit stupid to have to buy two straps just to get one wrist pin but can't fault the customer care team as they responded to me quickly and arranged to send me it out. I had 2 days later. Can't say much for their eco policy though!!