Advice navigating OEBB train or Trainline Site

I tried booking one on the OEBB site for May 7 with a change in Villach (seemed like the shortest train). Got as far as seat selection but wasn't able to select a seat on the second train for some reason...

This isn't a complete answer, but perhaps it will be of some help:

Although it won't sell you the tickets you need, you might take a look at the Deutsche Bahn website, which I find is often easier to use than some others. What I see there is:

8:12 AM: RailJet + RailJet, changing in Villach with a 7-minute transfer time

10:12 AM: RailJet + Bus operated by the train company, changing in Villach and terminating at Venice Tronchetto or Mestre rather than Santa Lucia, meaning an extra change to get where you want to be

11:06 AM: RailJet + EuroCity, with the EC operated by Deutsche Bahn, changing in Innsbruck with a transfer time of 18 minutes

I see no direct trains.

Sorry I guess I shouldn't have said completely direct.

The 11:06 train from Salzburg to Innsbruck and then changing to Eurocity train to go to Venice shows up on OEBB. It's a little bit longer than the Villach one, but I like the idea of staying on trains instead of a bus and the brenner pass better I think..

I just saw the two rail jet ones through Villach though. Hmm. That would get me in Venice earlier.

Also do you guys recommend paying for seat reservations for this trip, or just finding empty seats for 3 people should be ok?

On the seat reservations question, I always pay the small extra charge just for peace of mind -- have removed people from our reserved seats on occasion when they overlooked or ignored the signage above the seats that said those seats were reserved. (P.S. a seat is not necessarily "available" if it's empty when you get on the train -- it may be reserved for a later part of the trip.)

For the small amount of money involved, and you want to sit together, invest in seat reservations.

As far as not being able to get seat reservations on the EC connection at Innsbruck to Venice, that train is operated by Deutsche Bahn. You could spend another 4.50 EUR per person and purchase the seat reservations at www.bahn.com

Thanks for the advice on seats. I'll probably do reservations then for piece of mind.

Now I just need to decide if I want to do the train through the Brenner pass or the one through Villach.

DeutscheBahn does sell at least some of these departures and maybe even more than last year. Their remodeled web design is making you check prices specifically for the departure you want (after adding dates of birth). DB has usually been easier for credit card approval than is OEBB.

I just booked 3 tickets through OEBB! Decided to go with the route through Villach, but it's all by train, not bus.

I also reserved seats to be safe. I didn't get to pick specific seats though, it just says they'll select numbers based on my criteria. I opted for a table, so I assume that purchasing 3 tickets with a table that will assign all 3 seats together?