Other hardwoods can be tapped and their sap made into syrup, it’s just that maple is the tastiest, with the Sugar Maple being what we think of when it comes to making syrup. Birch and Walnut are probably the most common alternatives.
Funny thing, in my family we're all allergic to birch pollen. So before the pollen season we often tap some birch sap and drink to bolster our immune system in advance. Dunno if there's any science behind it, but in my experience it's done wonders.
It's a myth. For a start most people's hayfever isn't anything to do with flower pollen, it's grass and tree pollen and fungal spores. Pollen and spores can be carried by air currents and travel long distances. The flowers your local honey comes from are unlikely to be causing your hayfever. You should buy local honey over commercial honey though because it supports small producers.
Interesting — is the birch sap/syrup more allergenic than maple? I’m allergic to birch to some extent maybe more than other trees. But also I’m pretty sure I’m allergic to maple also (and many other trees) but eat maple syrup no problem.