Vetbed is better because it is soft and will keep him dry.If he is not moving the hay will get damp and the last thing he neeeds is to lie in a damp bed.
Ralfie may be eating enough anyway,and it is good that he is eating hay.You really need to weigh him,that is the only way you will know whether he is eating enough.I understand that you will be nervous about hurting him,but remember this is bruising,not fracture.I do not know how big the bowl on your scales is,but pad it somehow ,then check the weight,then gently lower Ralphie in.You need to check the weight morning and evening at the moment .,but if you feel this would be too stressful for him,do it once a day,in the eveining.
There is no need to towel wrap unless the pig is very strong and stroppy.I guess Ralphie will not be able to wriggle about too much.
There are several different ways to hold a piggy to syringe feed,you need to find the method that suits you and Ralphie.Every pig is different,so a method that works with one pig may not be any good with another.
One way I use is to stand the piggy on a firm surface at waist level,put my left hand under the pigs arms and pull him up so he is standing straight,and I hold him against me.For obvious reasons you cannot do this with Ralphie.
Another way is to put the piggy in a safe box or basket(open topped) on a table and offer the syringe,some pigs will just grab the syringe with no problem.
Another option is to sit with the pig on your lap(on a thick towel or vetbed in case he pees,and he probabvly will if he is being fed) then just offer the syringe.
Do not worry too much about any of this because it sounds to me as if Ralphie is doing well.
Bruising takes a while to come out. Arnica cream(from a healthshop) massaged into the limbs might help,it is a very old fashioned,but good,product draw out bruising.It is homeopathic so there are no chemicals in the cream.
Whoever said that guinea pigs are easy pets to keep,has obviously never kept them.!!,they are very difficult little whatsits.