Kiwi, don't let that rascal discourage you brother.
Your efforts/service will be rewarded.

1. Yes it's certainly worthwhile, if done properly.

2. Witnessing door to door is also effective, and can be done many ways, such as Gospel tracts, a simple presentation combined with a handshake or prayer, etc... also starting a Bible study during lunch breaks can be very effective, you can meet off property once a week and have a picnic with fellow employees. Home Bible studies can be an excellent tool for witnessing to neighbors and friends.

3. The only restrictions to "preaching" would be the limitations of the preacher. A man who is familiar with the Bible and dealing with groups/crowds can certainly pull it off, but a new believer who has little knowledge of the Bible could get caught in a web if there are hecklers present. Public speaking requires a certain amount of discretion, and a decent handle on communication skills. Not everyone has this calling. The best thing is to get involved in a local Bible believing church in your area and team up with an experienced mature Christian who can show you the ropes.