I've only just come on, & I'm gutted by this.
You can get the penicillin runnier to inject perfectly safely by warming it in the syringe in a mug of hand hot water - about 40 - 45C
To seal off the syringe I cap it with a needle bent back on itself. This is highly dangerous to you in terms of stabbing yourself, but the best we can do on a Sun. evening. Proper plastic syringe caps do exist.
(This is what we used to do for benzyl penicillin injections for humans years ago - it's like trying to inject thick honey at times)
The pain factor - with abscesses this depends totally on whether they've plenty of room to expand eg just below the skin. In those circumstances they're not very painful. When in bone, or where there's no room to expand, they're very painful as on the weight bearing part of the foot.
As for what to do, that is very much between you, your O/H & your vet, who know Grim & Smoo so well.
I think a big factor at Smoo's age is that she could have something going on of which we are unaware of eg a touch of arthritis & this has caused her immune system to plummet.
OK Benjie had 2 abscesses +snuffles when he arrived, but he'd just been through horrific bunny nightmare, & he was prepubertal. It was a totally different situation; very difficult, but with a lot going for us.
Whether they should go together is a totally personal decision. You have my total support which ever path you chose.
This has been my most difficult post. "Doing a Grimlock" is a household phrase for play, like he does with the washing & which both Thumper & Benjie enjoy. I've no words, just think that if I'm very fond of your 2 amazing buns, you must be totally devastated.