As far as substituting ingredients go, the most important thing is not to reduce the ratio of acid to other ingredients.  Messing with the tomatoes, vinegar, lemon juice amounts is a no-no.  What allows things to be "pickled" and so BWBd, is the acid to non-acid ratio, mess with that and you need to pressure can.

I always try to find other recipes in reliable places that are very similar.  Taking out peppers and replacing with onions is ok as long as quantities are the same.  I'm much more careful about replacing tomatoes because they are at least somewhat acid even though nowadays we add extra lemon juice to UP the acid because tomatoes often aren't as acid as they used to be.

Is that confusing?  It's all most important if you are planning on BWB (Boiling Water Bath) canning.  Pressure canning a mixture of anything means you process it for the longest time required for any one of the ingredients. 

A lot of people say things like, "Well it's canned in the grocery stores." well, if I had the equipment and testing ability of a large corporation, that would be different.  For me I stick to the basics at home, and really, that's just best for me anyway.  Basics in home canning can be very fun and fancy…  I just try to take out the risk factor.

Sugar?  That's a whole 'nuther topic.  I have no idea about replacing it with agave.  I find we can't use it so I haven't experimented with it.  Sugar is pretty necessary in canning jam or jelly, leaving it out changes the shelf life drastically.  Freezer jam is best if you're going to go really low or no sugar in jam.  My Pop is 93 and has to watch his sugar, but by going just a bit lower with the sugar, we find the amount used on toast etc to be pretty negligible and safe for him to indulge in.

I think in most things like pickles or relishes, it's more of a 'taste' issue than preserving, so it may be ok to replace it, I'd probably test it with the normal ratio used instead of sugar in a small batch.  Thing is, again, in the small amounts of sugar that ultimately wind up in say a tablespoon of relish on a hot dog or sandwich… if the agave doesn't work out I'd evaluate how much that bothered you. 

I hope that helps and that others will chime in!  There are some good, safe, reliable sources in our Canning Links section here  http://chickensintheroad.com/forum/the-canning-pot/canning-links/